I have to summarize what I am reading by putting each paragraph or two into my own words either in computer notes, notebook paper, or in the margins of the book (for books you plan on keeping, and yes, I know this is a sacrilege to some). I also can catch it immediately, if I don't understand something. That way I can I can clarify it in the moment by looking up more information online, as Perna suggests, and put this in my notes too where it is pertinent.

This takes more time but I also then have a condensed summary to review for tests, only having to go back to the book to look at graphs and images if applicable.

I don't know if this might help but knowing how poor my memory is, once I started doing this, as well as what Perna suggested above, made a huge difference in my retention.