[reading: Barton Zwiebach, "A First Course in String Theory"]
A couple of months ago I'd been reading a good book on string theory, but I stalled about halfway through. The book is aimed at undergraduates, so I could pretty much just read it straight through and check all the equations without needing pen and paper.
However, because I was reading it a bit at a time (typically on the train to work) and not taking notes, eventually the sum of all the bits of information that I understood at the time but have since forgotten reached critical mass and my rate of progress dropped to molasses-slow.
So now I'm going back to the beginning and taking notes as I go; when bookmark B reaches bookmark A, hopefully I can then carry on through the rest of the book. And then, when it's done, I can return to the other physics book that I've been (nominally) reading for an even longer period (like 2 years, or even longer if you include the attempt I made to read the first edition).