Books have to be my all time favourite gift idea. To make a family history come alive, the genealogist will research the history of the times and include the information in their reports. Often books featuring the era one is researching are out of print. I remember hunting far and wide for a book on Empire Loyalists and finally found it at an used book store.
If you are looking for a gift for the genealogist in your family, one source of rare and out of print books is Alibris.com. First editions and signed books can also be found at Alibris and the selection of >any< type of book (or music for that matter) is vast. It is a great place to buy a gift either for someone on your list or yourself 🙂