If you are interested in learning for retention (i.e. learning something and then being able to remember it the next week, next month, next year...) then you sh

If you are interested in learning for retention (i.e. learning something and then being able to remember it the next week, next month, next year...) then you should look into Spaced Repetition software (search for "spaced repetition" on Wikipedia for a good overview). The software presents information in a question/answer format, like flashcards, but spaces out the repetition so that the material you need to review most you see more often, and the material that you remember well you see less often. There are several programs available that support this form of learning including Mnemosyne, SuperMemo, and Anki. As for how I use the software, as I'm reading something I want to retain, I try to extract essential concepts as question/answers pairs and add them into a deck in the software. Then review the deck with the software on a regular basis (usually daily). The spacing effect provides maximum learning with a minimum time investment. As for choosing a topic, that's entirely up to your personal interests. There are no shortages of posts on Hacker News listing books the readers are interested in. Search Hacker News for "good books", go through one or more of the lists, and choose one (or more) of the books that sound interesting to you, and get started.