Good Books for Precocious K-3 Readers

Does anyone know some good, age-appropriate books for readers who are young in age but advanced in reading ability? I spent the last semester assisting in a 1st grade classroom, and one problem I encountered was that three of the kids were excellent readers (way beyond grade level), but we didn't have anything in the classroom that challenged them. I remember being in a similar situation in early grades, and my parents responded by just turning me loose in the adult sections of the library, but I suspect most parents won't agree with that approach, so I was wondering if anyone had some suggestions for good books that kids that age could read and still be challenged.