Or at least, you don’t have to read everything.  Here are two useful practices that can get you good information short of reading whole books:

• Read parts of books.  In particular, I think that you can read the first 50-100 pages of almost any novel and get a good feel for the book, i.e., you will get to know its protagonist, you will see how the author tries to make the protagonist sympathetic and/or hook you into the book, you will see what subplots are set up, how much detail goes into communicating milieu, etc.  Quick-partial-reading eight or ten books this way can really help you get a feel for a particular genre or subgenre you are thinking about writing in.
• Read bookshelves.  You need to be aware of the limitations of the shelf you are looking at (for instance, realize that library shelves will include older books that may not currently be selling copies, and you should know that bookstore shelf space is bought and paid for by publishers, so may not truly reflect natural reader demand), but browsing through a bookstore or library section, just briefly examining the individual books, can give you useful information.  It can help you understand what kind of thing your target audience likes; it can help you clarify your own thinking as to what your target audience is; it can help you figure out how to describe your project to industry professionals (“Well, Sharon, it’s basically CIRQUE DU FREAK meets HIS DARK MATERIALS, with a twist of THE HUNGER GAMES”).

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