Readability formulas estimate how difficult text is to read. The resulting "readability level" helps teachers and school librarians match students to appropriate books. Guiding students to appropriate-level books is now easier and more accurate with the ATOS (Advantage-TASA Open Standard) Readability Formula for Books, the new readability formula developed by Renaissance Learning, Inc. This report discusses the advantages and disadvantages of popular readability formulas, including a comparison of open and closed standards. The report also discusses some of the limitations of all readability formulas and how best to use readability formulas to help match students and books. Finally, it describes the new readability formula, ATOS for Books, and how it overcomes some of the limitations of older formulas. (Contains 4 tables of data and a 15-item bibliography.) (NKA)