Or maybe just two - depending on the cost. I want to buy these as an engagement gift for a very good friend. She and her fiance are vegetarians who do eat fish, so it sort of doesn't make sense to get them great cookbooks that are 75% meat-oriented recipes. On the other hand, I'm not so taken with any strictly vegetarian cookbooks that I've seen, so I'd prefer something a little broader. They're adventurous eaters and love love love food.
Here are the types of books I'd be most interested in getting:
Something Middle Eastern or Indian
Something all-purpose (like a Bittman book)