Here's what's great about Lexile levels--you can combine it with other reading level measures, like the ones on the Scholastic site, and come up with books appropriate for younger kids but that have more complex language. For example, a book that has a reading level of 3.9 but a Lexile level of 900 is going to be more challenging to an advanced reader than one with the same reading level but with a Lexile level of 400.