I abhor highlighting, underlining or writing in books by some previous "reader" as well as by myself. Why? It adds emphasis or de-emphasis which impedes and usually eliminates the process of rediscovery and potentially seeing something in a different light. I prefer my reading to be dynamic and unbiased by other people as well as by myself, whether that's from a few minutes ago or a few years in the past. Commentary, which is often situational and ephemeral, should be separate from the original. When a book is marked up, your choice to ignore or seek commentary is taken away.