This is tough. *only* three books? argh! But I will try really hard, just because I like you guys.
My collection:
1. BOS (since my BOS is a ring binder with both blank and written pages in it, I'll just cheat and assume that the paper requirement is covered)
+ and many pens
2. The Magician's Companion (I forget who wrote it)
3. The Herb Book by John Lust
If I could scam a fourth, it would the the Inner Temple of Witchcraft, or maybe The Wiccan Warrior.
Um. This is the reason I don't like desert islands. Not enough bookshelves!
Oh, and I'd want at least one tarot deck. OR maybe I'd use runes instead, since they are much less likely to mold or something dreadful. I guess I'd hope that there was some clay on that island, so I could use cuniform or something for continued note taking. But I'm not sure I'd have time for that much transcribing before the books themselves would go bad!
Enlightenment: I'm going to a con in a couple of weeks and meeting Christopher Penczak!

PM me if you have any requests! That goes for all of you.