I find the act of reading a book from start to finish over a period of time is a good deal. There is definite progress toward a goal, you generally learn something and books are portable and generally cheap.

It's also a decent way of exploring something without distraction and to a guided level of depth. Usually by the end of it you have a feel on how deep you want to continue and what aspects you don't understand. Depth vs breadth is usually more easily traded off each other after you've read one or more books on the subject.