If you have a particular book in mind, Amazon.com usually has that information under Product Details.
The library catalog does not allow you to search for books for a certain age or reading level. This is because every child generally has different reading levels and because not every publisher provides this information.
The best way to find a book for a certain grade level is to have a book already in mind, have a subject in mind and then narrow the results down by looking at things like page length and description, or to browse the shelves.
Publishers will sometimes indicate reading levels on the front top corners of a book or on the back cover near the corners or barcode.
