Every once in a while I’ll get a query that includes descriptions of two or more books, or because authors are told to query only one book at a time, back-to-back queries from the same author. Let me tell you why neither of these is a good idea.
When I read queries I’m looking for a reason to reject the book, I’m asking you to prove to me why I should do otherwise. A query for two or more books (or multiple queries) not only gives me more choices to reject, but makes the book I might be on the fence about an instant reject, since I don’t have any interest in your other work.
If you want me to consider the body of your work then I’m going to need to love every single piece in that body of work before I’m going to ask to see more. If you’re a client, we can have more concrete discussions about how your body of work plays into your dreams for a career. When you’re a client, I might not choose to represent every book, but after some conversations you might not want me to (you might see why some books should stay under the bed). Or, because you’re a client, I might be more willing to spend time with you on the books I wasn’t initially that excited about to build them into your career.
One book at a time please. If you get rejected, feel free to try the other, but don’t fill up an agent’s inbox with all of your work. We need time for others too.