I used to revere books, but after buying one too many that would have been better at article length (and often started out as such), I've become more guarded with the time I'm willing to invest in a book. Some books are worth a re-read, but an increasing fraction that I encounter now aren't worth a first.

"The 48 Laws of Power" is an example. I originally read this as a magazine article, which was great. I excitedly bought the 452 page book of the same title when it came out, but unfortunately it didn't cover much more than the magazine article I had already read; it was just book length.