This is some of the best advice available on HN.

Don't (just) read books. Put down the highlighter. Explain them. Get in front of a whiteboard and explain it to an imaginary audience. Imagine the questions you get back. Try to answer them. Sometimes you can do that nearly sentence by sentence, other times you have to do it section by section.

I was lucky enough to have a Feynman level (in terms of teaching) EE professor. I banged my head against modelling electron behavior in NPN junctions until I recognized that his teaching style was exactly how I needed to approach the material. From there on out everything became much easier.