Most books are terrible, and even most "good" books have large swaths of filler content. Very few books are packed with useful content from front to back cover.
I think this is largely due to pressures from publishers and the way books are traditionally published and sold.
A 50 page book that is jam-packed with goodies and has no filler won't sell nearly as well as a 300 page book that seems to be full of stuff from an expert in the field. A 300 page book looks a lot better on a shelf than a 50 page pamphlet. Most of us know in our brains that quality > quantity, but our hearts often tell us the opposite.
Some fixes already exist: Online content, blogs, developer docs, and self-published books.
Practically, I would say don't worry about skimming books or parts of books if they seem like they are mostly fluff or overly repetitive. Because they probably are.