I enjoy a mathematics textbook as much as the next person, but what annoys me is the lack of solutions to the problems in so many of the books I've skimmed outside of classes.
That's why Khan Academy is awesome. I'm working through their calculus and differential equations curriculum right now to refresh my math and the amount and quality of autograded excercises is fantastic! The other nice thing is it periodically tests knowledge of previously studied material.
I'm doing ECE at university now and I have an exam in about 7 weeks. I've found that it's quite hard to identify what sort of differential equation a given problem is, and then how to solve it. However I've found that by looking around for other material I can understand it better.
For example web searching "type 4 differential equation" bought up this gem: https://www.math.ucdavis.edu/~thomases/W11_16C1_lec_1_7_11.p...
I think I'll try out Khan Academy because I've found that the lecturer's problem sheets probably aren't enough on their own.