I write in them too. Not all of them, though - mainly I write in the ones that I either really, really like (and thus want to highlight my ultimate favorite passages of all time in, so that I can more easily relocate them later and quote them to anyone who will listen), or that I really rather hate certain small details about, but still feel that there's enough good in them that the author ought to bother improving them (then I start making editorial notations about the awkward sentence structures or biased language).