Reading is a regular occurrence in my nonlucid dreams and is generally no problem when lucid dreaming—I do it without a second thought. I can't remember if I've ever tried doing extensive reading while lucid (I know I do it all the time in NLDs), but mostly I read brief labels (e.g., on buttons and controls of unfamiliar devices or machines I'm exploring) and things like that. I would expect long passages of text to behave like verbal dialogue: generally being grammatical, but maybe or maybe not making much sense in overall context.