I've found that the process of creating the cards is helpful because it forces me to make the book's major insight explicit. I usually use cloze tests to run through a book's major points. For example, my card for The Lean Startup is:

"The Lean Startup process for continuous improvement is (1) {{c1::identify the hypothesis to test}}, (2) {{c2::determine metrics with which to evaluate the hypothesis}}, (3) {{c3::build a minimum viable product}}, (4) {{c4::use the product to get data and test the hypothesis}}."

This isn't especially helpful if you just remember what the four phrases are, so I use this as a cue to think briefly about each of those concepts.