I know many people are confused about library practices -- especially when they find perfectly good books being sold at a library book sale. Why would the librarian remove that book from the library?
(My usual explanation is that new books are arriving every week and if an older book has out-of-date information or is worn out or nobody has checked it out for a few years, then it needs to be replaced to make room for the wonderful new books.)
Well, here's a great article by a Young Adult librarian at the blog Stacked explaining the Art of Weeding -- What her teens are not reading. And why.