Have you ever tried to keep one?
I have attempted to keep a commonplace book related to my reading habits three separate times--and abandoned it twice. Not really a reading journal, but notes, quotes and page numbers to make it easier to go back and find certain passages that struck me as memorable or noteworthy.
For most of my life I was--for lack of a better term--a passive reader. I read, and read, and read some more, trying to soak it all up with nothing but my meager brain. When I was younger I had a half-assed theory that if I could cram enough knowledge into my mind, no matter how profound or puerile, eventually it would all click together and I would have, if not The Answer, at least a Pretty Good Guess At The Answer.
I gave up on this theory some time ago, but I am not so passive a reader as I once was. Now there is almost always a small notebook and a pencil near at hand. When I find a passage that strikes me, I write the page number and a word or two in the notebook so that I may find it again later. If it's a book that is already marked up, torn or otherwise worn, I might also make very light pencil marks in the book to bracket the passage for easier rediscovery later on.
There are some disadvantages to this. It may interrupt the flow of your reading. It may simply become irritating to stop and write a quick note now and then. I do not force myself to follow any strict rules regarding this; sometimes I wait until I have decided to stop, then quickly scan back over what I have read and make my notes. This kind of reading may not be for everyone, but for myself, I believe it pays off in the long run. It helps me to better understand and remember where I've been, and I think it's better to read a little and understand a lot than to read a great deal and understand very little.
And of course, if you plan on keeping a book or reading blog, a commonplace book may prove to be quite an essential item.