Barnes and Noble now lets you search children's books by lexile ranking. Lexile ranking is the reading difficulty of the book, separate from grade level. A child's lexile level is determined by reading comprehension tests, and texts are analyzed for word frequency and sentence length to estimate their approximate level.

The lexile search can be found here. The wizard works pretty well, allowing you to either estimate the reading level by whether your child finds grade-level books easy or difficult, or search by the actual lexile ranking if you know it. It then gives you a sample text to read, allows you to narrow by the child's interest, and then gives you suggestions.