I read cozy and historical mysteries, a bit of Paranormal/UF, and to mix it up, I read science and gardening books on occasion.
I'm going to have to cheat here and not only include two books but one I've already used.
Because desert island is all about the re-read. What book can you tolerate re-reading ad infinitum until the day comes that you are either rescued or taken to the Great Library?
For me, The Complete Sherlock Holmes and Pride and Prejudice cover all the bases: mystery, mental puzzles, action, a tiny bit of horror, humour and romance. And I can't imagine every tiring of either one.