Cultivating a genuine curiosity for the topic, for me that means be driven by my own questions in the first place. A side project could be a form of a question, don't feel guilty for not finishing those projects, some aspects of the project will answer your question and the other uninteresting parts will be never finished, and it's fine.

Also I used to read books linearly and the whole thing, recently I'm experimenting with these other ways from How to read a Book pdf[1] which I saw posted on HN[2]

[1] http://pne.people.si.umich.edu/PDF/howtoread.pdf

[2] https://news.ycombinator.com/item?id=12209446