When I read I always underline lines that are significant to me or that I want to try to recall.  When I have a speaking opportunity coming up I will pull out some of the books that pertain to the subject that I am to speak on and skim back over the underlined parts to get ideas and illustrations.

Once I have read a book,  I will keep it around for a while before placing on the bookshelf so that I can pick it up a few times and look at what I have underlined.  This practice increases my retention of the information significantly.

Also, I never feel obligated to read a book from cover to cover.  I have found that with most books the general premise of the book is revealed early.  Much of the remaining information is simply supporting the major premise (of course keep in mind I rarely read fiction).  If I read several pages (maybe 10 to 20) and have not underlined anything I generally conclude that I have read enough. 

I try to read a book a week but generally read between 40 and 50 books a year.  I probably read about 30% of those books cover to cover.  I probably read over half of another 50% of the books.  I read less than half of the remaining 20% of the books.

If I am reading a book that I do not feel is increasing my awareness or knowledge on a subject matter or perhaps the book is not inspiring me in some way, I throw in the towel and move on.