I finished my engineering career (Electronic Engineering), but I feel that my mathematical knowledge is deficient in important areas (like Fourier series, Laplace, differential equations.. etc).

I'm willing to learn all over again mathematics, but I don't know where to start. Actually did try to understand differential equations on my own but failed miserably.

How can I learn basic and advance mathematical subjects? Any recommended books, websites etc.?

Thanks a lot, and I apologize for my bad English and maths ignorance.