I will second/third/etc. the suggestions for Singapore Math and AOPS. A lot of the homeschool math books can be a little "light" so be careful about what you choose as your spine. Some of the lighter books are good for back-up, though. Somebody mentioned Zaccaro books. They're great, and were really fun enrichment books in the elementary years. My son loved solving the "Einstein Level" problems and receiving a little award when he solved one. I heard the author speak at a gifted conference once when my son was little and I found him to be really inspirational. He stressed the importance of giving our gifted kids real challenges.
