Having a local copy ensures you have access to the material regardless of your internet connection.
If your goal is to truly understand how deep learning works—rather than just copying and pasting code—Michael Nielsen’s book is the best investment of your time. Whether you read it online or via a PDF, it remains the most lucid introduction to the mechanics of artificial intelligence. Having a local copy ensures you have access
Nielsen provides "warm-up" exercises. Even if you aren't a math whiz, try to follow the derivations; they are where the "aha!" moments happen. try to follow the derivations