But Excuse Me, That Is My Book by Lauren Child.
I have always been charmed by Charlie and Lola, but this book goes right after my librarian heart.  Lola has a favorite book, Beetles, Bugs, and Butterflies, that she checks out from the public library every time she visits.  For her it is not only the best book in the world, it is the only book.  But this time, when she gets to the library, the book is not on the shelf!  What oh what is a little girl to do?  Well, Charlie steps in and does the perfect readers advisory interview to find her a replacement book.  Lovely stuff that will warm the heart of even the most serious reference librarian.  Oh, and kids will enjoy it too. It immediately got the children I shared it with talking about what their all-time favorite books are.