How can I read technical books and actually retain what I learn?

Reading alone is usually too passive for technical subjects. After each chapter or concept, try explaining it in your own words, sketching how it works, and implementing a small example. The exercises and projects associated with the material are especially valuable because they force you to use the ideas and reveal what you don't understand yet.

You don't necessarily need to highlight everything or write detailed notes. First read through a section to become familiar with its vocabulary and main ideas. Then return to the difficult parts, make brief notes, and solve the exercises without looking at the answer. Connecting the same concept across different books, lectures, and examples also helps build a stronger mental model.

A useful routine is: preview the chapter, read a small section, close the book, and recall the main points from memory. Then test yourself by answering questions or writing a tiny program. For operating systems, try building the suggested projects or experiments before moving on. Learning usually becomes much clearer when you encounter the practical problem that each concept is meant to solve.