In the past, I've recommended several books that can fill you in on the book publishing business. Of course things change and books may be outdated--and there's always a question or two that the books don't answer. So let me introduce you to an interesting source on how publishing works from the publisher's perspective, with things an author needs to know. It's current, and faster than reading a book.
The source is at Joe Wikert's Publishing 2020 Blog. Joe is a Vice President and Executive Publisher in the Professional/Trade division of John Wiley & Sons, Inc. (The company that published my book, Blogging Heroes.) In four posts, Joe answers a lot of questions about book distribution, marketing, sell-through (what's that?), and sales expectations (how do publishers project sales?)
Here are labeled links to each post:
Marketing and PR