+1 What I find really useful about these books is they allow self-teaching. (in my case a 13 year old kid) The books follow a methodology of presenting problems

+1 What I find really useful about these books is they allow self-teaching. (in my case a 13 year old kid) The books follow a methodology of presenting problems and having you try to solve them before walking you through the explanations. The chapters build up to "challenge problems" which are carefully crafted to require you to apply what you've learned from the current and earlier chapters. Most of the challenge problems are lifted from math competition problems (with citations), but their use is very well curated. We are using the printed books, but there are online resources available.