I am learning some pretty disconnected concepts of mathematics for a project I am working on. I was reading about matrices and suddenly had to know about Markov Chains and that lead me to read about Multivariate Expectations and so on.
I was initially spending time in the library with the main book and everytime I encountered a problem, I'd go looking for a book which I thought would explain it, read it and return to the main one. Just as an experiment, I tried using Wikipedia to understand the concepts and then did google searches with 'site:.edu' references to look for coursenotes and then browsed through some of those till I felt comfortable and went back to main book.
For mathematics, is Wikipedia a good first line of attack?