I am often pitched projects where an author will describe the book as a cross between several different categories. I usually find this problematic for the simple reason that a book that is described this way often suffers from an identity crisis, and publishers want to be able to clearly identify how a book will be positioned, marketed, promoted, and at its most basic level, where it will "live" in the bookstores (this despite the fact that authors often complain that their books are not available at bookstores anyway, since they can't possibly carry everything, and there are so many outlets now outside of traditional stores to buy books, but that conversation is for another blog post). For the most part, at the chain bookstores, books are shelved in one place and one place only, and it's not always where the publisher or author want it to go. If it's a cookbook that's also a self-help book that's also a memoir, or a literary historical romantic mystery, well, that makes it a lot more difficult to place.
I recently found this piece that talks about the different genre categories in fiction. It doesn't address nonfiction, which has its own language of categories, but it does serve as a good basic summary of the major fiction categories. There are always subcategories within each of these, and she leaves out thriller, which is often confused with mystery, and which I see as a subset, but really is its own category. The takeaway here for me is that whatever category your book falls into, you need to do your research into the best books of that category, where you can find them in the bookstores, and also which agents represent those books. That's who you should target first, and your pitch letter should be clear about which category your book falls under because if it's not, or it's a mix of too many genres, it's easy for the book to get lost before the reader even gets to page 1.