You can self-study pretty much any of the technical fields.
What do you want to learn? EE has a lot of subdisciplines and related disciplines, including information theory, rf circuits, analog circuits, signal processing, digital logic, optoelectronics, power, and motor design, among others. I may be able to recommend resources if you tell me which of the above you are interested in.
At a more basic level, you should be familiar with linear circuits, basic calculus, ordinary differential equations, and boolean logic. Once you understand linear circuits, you should learn about circuit analysis, such as from Sedra and Smith's microelectronics book. It is also very helpful to study linear algebra.
If you wish to get a feel for designing circuits without going through all the theory first, I highly recommend the book "The Art of Electronics" by Horowitz and Hill.