I have been tracking how many words I read.
I want to track something that I can measure without too much difficulty for both physical and digital books, and it should be general so I can (more or less) compare a small book with large text and a large book with small text.
For physical books I estimate the number of words on a typical page before I start reading, and then I just multiply. For ebooks I can generally just mark the text that I have read and share or copy it to a word counting app to get the exact word count. (I use an android tablet)
Apparently some Kindle ebooks has limits on how much of a book you can copy within a certain amount of time, (eg. 10% of a book per day or something) but I haven’t had serious problems with that yet. I have read somewhere that a kindle position corresponds to 128 bytes of text, so maybe we can figure out a typical conversion rate between positions and words. It would depend on the encoding, but I already have word counts for 1½ kindle ebooks, so I can probably get a decent estimate.