This site by Scholastic gives your an understanding the differences between leveled reading systems and choosing a system to use, plus how to make a leveled book list and what to do if a book isn't leveled. Also available are lesson plans, teacher took kit, book talk and author ideas, student activities, and a book finder page that lists reading levels, interest levels, and lexile for most books-- including a keyword search option to find books on a topic.
Lexile.com is a site that matches texts with lexile numbers, and includes a title search to determine lexile levels for books.