In another step in my quest to obtain more and more mathematical know-how I discovered the website http://www.askamathematician.com/about/ where mathematicians and physicists answer questions they received via email or personally. What makes the site very unique is the structure of the very thoughtful and detailed answers. Most questions are answered in multiple steps (like “the clever student would answer / the cleverer student would answer / the teacher would answer / the mathematician would answer” ) and small discussions which lead to the final solution or opinion.
In my opinion it is better than any math book because you’re directly confronted with an interesting question and can follow the answers step-by-step having time to form your own solution or opinion on the topic. This induces much more involvement than just consuming a book’s content. The consequence is that you really learned something after working (and I really mean working and not just reading) through the answers.