Maybe he just likes working through text books thoroughly? I personally find it quite valuable to compile a list of "outstanding questions" for the text books

Maybe he just likes working through text books thoroughly? I personally find it quite valuable to compile a list of "outstanding questions" for the text books I read. Usually most of my questions are resolved before I finish the book. The ones that remain, often turn out to be simply errors. Finding the the errors is just a by-product of the learning process for me.