Mark them up, scribble (digitally) everywhere, and bookmark to your heart's content. There is no worry about damaging the book or folding corners.
You presumably wouldn't steal physical books (apart from perhaps a once borrowed, accidentally now forever yours loan) but ebooks are digital and [[!Piracy]] happens all the time. PDFs and [[!DRM]]-free books are shared freely online (See: [[!Sharing vs stealing digital goods]]).