Some of my favorites: anything from Gluten Free Girl, Shauna Ahern; Gluten-Free on a Shoestring; or Gluten-Free Baking by the CIA.
Another thoughtful gift for a GF baker: an assortment of GF AP flours. Bob's Red Mill has one; there's Pamela's; King Arthur and Cup-4-cup are all possibilities. They all perform differently (slightly different taste and texture results), so it could be fun baking to try them (I once did side-by-side taste testing with each in my favorite chocolate chip cookie recipe. The only difference in the recipe was the flour used and the results were noticeably different).
I know that there are a lot of gluten free cookbooks out there. If you use Thomas Keller's "Cup4Cup" gluten free flour, you can make anything without having to fuss with a special cookbook.
Against All Grain by Danielle Walker is one of my favorite cookbooks. It was just released this year and everything I have made has been delicious.
I really like "Gluten-Free Baking" from Rebecca Reilly. Excellent shortbread cookies, and everything I've made from this book has turned out really good.