Pet peeve #3.1415926535 People who write in / underline / highlight library books. Fortunately→[More:] you rarely see any writing/underlining/highlighting past the 10th page or so. Apparently, all the this-is-so-important-I-need-to-underline-it-and-highlight-it-or-else-I'll-forget-it passages are at the beginning of books.
Either that or they have short attention spans. Or just aren't bright enough to read past the introductory chapters.