It's also my main problem with many otherwise great non-fiction books. First, it takes more time. Second, when I don't learn new things my thoughts start to wander, and I get very far from the reading flow state. Third, I have a feeling that either the author think that I am dumb so (s)he needs to repeat things, or it is hastily written (for me, an admirer of the Paul Graham's concise style, reducing text takes much, much more time than just writing it... and it's totally worth it).