> There are a lot of things publishers do behind the scenes that no one thinks about.That is definitely part of the publisher's pitch, but from what I gather, that is less than they'd like you to believe.
> Content editing.
Good editing is invaluable, but it's important to distinguish different kinds of editing. "Developmental editing" is high-level "what kind of book does the market want us to write" guidance. If you're writing a technical book, you shouldn't need that. You should already be plugged into the scene for the people who want your book and know what they want. Your domain expertise should also mean audience expertise.
An editor at a publisher doesn't have your domain expertise, and probably doesn't know the audience as well as you do. When I wrote a book on software architecture for games several years ago, an editor at O'Reilly wanted me to use Objective-C as the language (instead of basically pseudo-codish C++). Think about how much I would have regretted that choice today.
Line editing and copyediting are making sure your stuff is grammatically correct and consistent. This is really helpful, but you can also simply hire a freelancer to do it.
> Publishers often hire another person who is at least or more knowledgeable on the subject to review what the author says.
They pay technical editors a pittance. (I know, I've been asked to be a technical editor for several books.) Those that have the deep skill you want are too busy and worth too much to do it. Those that say yes probably won't do as good a job as you'd hope. It's simply not worth their time to go through your book with a fine-toothed comb. They'll find a lot of stuff, but you really can't rely on them. And, again, this is something you could hire yourself if you wanted to.
For my two books, I simply made the repo for them open and let people file bugs. That has been much more thorough than a couple of busy technical editors would be.
> There are other things too like a content editor. Does the entire book flow together (2+ authors, that's a big deal). Is the language style the same? Are there spelling and punctuation errors? Is the formatting correct?
Yeah, this is important. Though it's worth thinking about how much you're willing to pay them to do this. If it's worth it to you, great. But it may not be.
> Someone has to build the index at the back of the book. (Sometimes the author does this).
Yeah, indexing is a chore. You'll do a better job than an indexer would because you know the domain better. You can also, honestly, just kind of half-ass it. Indexes are less important now that you can search in ebooks.
> Not to mention, the company puts their name behind the book. I buy a lot of books simply based on the publisher and pass on others. As an author, you're paying them to lend their reputation to your work.
This is true and is a big one. The big publishers have done a great job of building prestigious brands and they loan you that prestige. That can be very valuable. This matters in particular for fiction where there is a sea of garbage. With technical books, it's so hard for an author to finish a book that I think it's less of an issue. Also, frankly, I think a lot of publishers have been diluting their brands by publishing stuff that isn't that good.
Either way, readers will come to your book largely because of the topic and hopefully because they know who you are already. A pretty woodcut animal and O'Reilly logo on the cover only goes so far. How much is that worth to you?
The way to look at this is: will a traditional publisher increase my sales enough to more than pay for the share they take? If so, do it. If not, they're just leeching off you.