I will 2nd this and what mathwonk said. When I don't understand something (which is often
) I go with a 'many perspectives' approach. I pick up 3 or 4 books (from the library since I am not rich) on the same subject and take a pencil and paper and try to re-derive what each author is saying. One author might go with a 'formal' derivation, where another might go with an 'intuitive' or 'pictorial' derivation and I make it my job to reconcile all of the approaches.
The internet is also a great resource as is this forum. If you have questions about calculus homework, ask in the Homework Help forum. If you have conceptual calculus questions, ask in the Calculus forum.