In my role as a school librarian, I regularly am asked - by teachers, school counselors and students themselves - to provide books that fill this role in a small way, for example recommending titles about the death of a pet or conflict with a friend. More often students' own choices of books are influences - consciously or not - by this need.
There is huge comfort in picking up a book and seeing your feelings mirrored in it. It is a comfort to know you are not alone and to see how fictional characters deal with their situation. School Library Journal recently published two excellent bibliotherapy book lists for teen readers - check them out!