Maggie Lynch's helpful response to this question on an Authors Guild discussion forum (published here with her permission):
Do agents prefer manuscripts that have been reviewed by a professional editor?
Before I start sending out query letters to agents, I'd like advice on whether it's worth the expense of hiring a development editor to evaluate my manuscript. I've received positive feedback from two beta readers, and I feel like the book is ready to go after 5 years work on it.
Congratulations on finishing your book and getting ready to query. Your question generates as many "well, if...." possibilities for a response.
My first question would be who were your two beta readers? Were they friends? family? writers? Do they normally read the genre you are writing in? Did you pay them, or did they do it because they like you and want to help you? Though agents and publishers want a "clean" manuscript, what they want most is a great story. A story that will get readers to buy even from an unknown author. A story that is so well put together, organized and sequenced to make sense, and keeps the reader wanting to turn the page and stay with the book--even if it means missing sleep. They want story more than perfect grammar or lack of typos. Editors will tell you: "I can fix grammar, typos, a few sequencing problems. I cannot fix a book that has story problems. I cannot fix a book that doesn't hold together." They actually can fix it, but it takes too much time and it isn't worth it when they have hundreds of other people's manuscript waiting in the inbox.
I ask WHO read your book because the vast majority of other writers, friends, and family are not good at providing feedback. Many writers, even really good ones, often don't know why their books work and can't articulate if yours does or not. Family and friends, even if they do read in your genre, aren't likely to say anything bad. Also, they don't necessarily know what makes a good book. They can certainly say they like your story, or they didn't catch any typos or grammar issues, or they cried or laughed. But can they talk about how it compares to other books in your genre? Can they talk about the pacing, the characterization, the descriptions, the plot and subplots, the themes, and how it all hangs together? Can they tell you what the liked or didn't like about your characters and how they moved through life, solved problems, recovered from trauma or whatever they needed to do to get to the end?
Let me share a recent story of readers for my current work and how not finding the right person--even in terms of professionals--makes a difference. I have 27 books behind me, a combination of fiction and nonfiction. So this isn't my first book. However, all of those books have been for the adult trade market. I do have one fantasy series of three books that has been marketed as YA, but the actual readership is 50/50 adults and YA.
My current book is a Middle Grade contemporary fiction book. I know I'm a decent writer. I know I can tell a story with emotion, and my pacing is usually good. I have a reputation for writing characters readers can identify with and remember. However, no matter how much research I've done for the MG market over the past year, or the number of books I've read in preparation (more than 40 books over the past year), I knew I might not hit it just right. In many ways it's like starting over with a new genre. I was concerned about pacing. I was concerned about language. I was concerned about chapter lengths, and overall structure. I had read a lot of MG books that could be comparable in terms of themes, age range, and resolution. But...no one wrote exactly what I wrote. No one book put all the things together I did. So...did I get it right?
First, I sent my finished manuscript to two professional editors I know and trust. They both had minor things to change in the story (e.g., more pathos, cut a scene that doesn't really advance the journey, better description, etc.) Both said they loved it and thought it was written at the right level for 11-14 year old children. BUT neither of these editors had ever edited nor written a children's book before. Yet, I was buoyed by their comments and made the changes I agreed with that each suggested. Before I started querying I sent it to a professional editor who has edited hundreds of MG books both for authors and publishers. She herself has written, or co-written over 100 MG books. BUT the vast majority of her work has been in nonfiction. She also told me that she loved the story. She concurred that it was written at the right level for language and understanding the themes. She liked my protagonist and the ending. There were some minor copy edits but she had no feedback on major story issues.
Then I started querying, nearly 100 agents over a six month period. I had a lot of form rejections, a few "I'm too busy and have stopped reading." and three requests for fulls. The three fulls didn't reject me, but they didn't contact me either.
I started to wonder what is wrong? Is it the subject? The themes? or is it something basic that three professional editors didn't catch? For two months I sat on it, unwilling to seek out yet another editor. Is it "good enough"? Probably. Could I get it published through a small press? Very likely given my track record. But...I wanted it to be right and to be picked up by a good press with a good reputation, and really good distribution. It doesn't have to be a Big 4 publisher--I doubt it's commercial enough for them. But I don't want just any press that will take it, I want it to be with someone who could do more than I could if I self-published. I needed to know what was standing in the way because I'd planned a series based off this first story.
A month ago, I decided I would seek out someone who writes Contemporary MG, has explored some of my themes, and is a well-respected editor or book coach. I found someone who was a National Book Award finalist with a YA book and is a bestselling author in both MG and YA. I paid for a beta read with feedback. That was a hard bill to pay. I didn't really need or want a copy edit or a developmental edit. I just needed to know if something was structurally wrong.
There WAS something wrong. Something none of the other three editors had noticed or commented on. This person also thought I was a good writer, she liked my themes and my protagonist, but... she could tell I was writing for adults. My language level was good (except in a couple of places). My overall plot and characterization was good. But there were three things I was missing.
1) I spent too much time in some scenes with other characters (e.g., friends at school) that were part of my protagonist's orbit but didn't need as much attention. Those scenes could be summarized in a paragraph or two. She explained that MG books need to stay with the protagonist's primary needs and story and not spend too much time on "walk-on" characters (my words not hers).
2) I needed a little more time spent up front in what is often called "the ordinary world" (Joseph Campbell's Hero's journey model) before everything changes. I thought I had done that in my first chapter, but she felt I needed a little more for the MG reader. I moved too quickly to the change (chapter 3). Again a difference between writing for adults and MG.
3) I also needed a couple more scenes at the end of the book to assure MG readers that things are working out. Most adult book editors often cut scenes after the "real" ending because adult readers know if you end with a promise of hope it is enough. (The one exception is the romance genre where often the HEA is assured with a marriage or an epilogue to show it's all wonderful). She explained that MG readers need that reassurance. Unlike adults, particularly in a book like mine where my protagonist has overcome a lot, hope is not enough. You have to show it is working--provide evidence things are going well and the protagonist is thirving.
This feedback is invaluable to me. I'm confident it will make a differences in the book finding the right audience, getting better reviews, and ultimately selling better than it would have. I also feel more confident as I begin approaching publishers in the new year.
Even though you are not writing a children's book, my point is that every genre has expectations. Every genre has things that readers look for--including literary and memoir. There are threads that need to be followed. The way you choose to approach a book and the themes need to be make sense and hold together. The language you use and the sequencing you use are all important. ALL of these things are rarely noticed by readers--even good readers, and even more rarely articulated. BUT they can be the reason a book isn't picked up after the "look inside" or gets tepid reviews from readers or the trades. Or doesn't get chosen for a review. The reader only knows they didn't like it, or it didn't hang together, but can't really say why. Perhaps an overriding metaphor didn't work. Perhaps the timeline was confusing. Perhaps the protagonist didn't make the reader care enough to finish the book.
If the people who read your manuscript don't know what those expectations are, you may very-well be let down when you start getting rejections. This is especially critical if you are planning to be traditionally published--whether with an agent or without. Unfortunately, genre rules are not cut and dry. There are many ways to tell a story, many topics that can work, but only someone who has read extensively in the genre knows where you must adhere to the rules and where you can stray based on your topic, writing style, and the market. It's not that you have to write a cookie-cutter book. But, if you are going to stray from the expectations, it has to be that much better than most books on the market.
You may be the outlier who is an amazing writer, with an amazing vision, and naturally has pulled it all together. I've known a couple people like that, but it is rare. For myself, I don't count on being that outlier.
Having the right person read and give you feedback can make the difference between having a chance at an agent or trad publisher and not. Even if you do end up going the self-publishing route, you will know that your book is the best it could be in that moment. It's hard to put out that money for a professional. It's equally hard to take the feedback and make it right. But IMO, it's well worth it. This is the beginning of your career. Why not start it right?