How regularly do you apply what you learned? Of the last 10 books you've read, have you used something from each of them?
If you're applying what you've learned, and it seems to work, that's how you know you haven't lost comprehension. If you're not applying what you've learned, it doesn't matter that much either way.
Most of what I read isn't books but internet discussion. It's often hard to distinguish what using an intellectual idea means. I do learn things and bring them up in a separate discussion a year later.
If I reply to your post than I'm using the fact that I read your post to write an answer. Better understanding of your point of view translates into a better answer.
Previous Open Thread: http://lesswrong.com/r/discussion/lw/k94/open_thread_may_19_25_2014/
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should start on Monday, and end on Sunday.
4. Open Threads should be posted in Discussion, and not Main.