In the course of the novel, we find out a lot of things, including how to write a dirty book. There's a guide to the four basic plots, and there's even a complete outline for a book, except for the last chapter, which is, shall we say, a bit exaggerated.
Some have seen the book as a sort of roman à clef, and if you're familiar with the agency Westlake worked for and some of the people associated with it, you can play that game. I don't know if it's worthwhile, but, after all, the narrator's name is Ed, and Westlake did write as Edwin West. (Edwin was his middle name.) It's funny and sad and even a little bitter. Certainly Westlake didn't wind up like Ed Tipliss, though maybe he imagines that he could have.
Decent copies of the edition pictured here are a little hard to find these days, but you can get a beaten up hardback on abebooks for about five bucks. It's worth it if you just want a reading copy. If you read French, you can pick up a nice copy for only a little more. But if you just want to know about how real it all is and to learn something about Westlake and the other writers who produced the kind of books Topliss wrote, what you really need to do is to read this great essay by Earl Kemp. Kemp quotes liberally from the novel and it's all entirely fascinating. Trust me.