The fastest way to learn is always to go after a difficult problem you find fascinating. Learning by the books is the second fastest, and is suitable for those without the experience to have a fascinating problem to motivate them. This applies to many but not all, for example a university student might have a vague interest in AI but not know where to start. If you’re already in the field though, it’s best to just keep hacking forwards, even if you can’t prove every theorem from the major textbooks.