This document discusses the measurement of reading ability and the readability of books by application of the Lexile framework. It begins by stating the importance of uniform measures. It then discusses the history of reading ability testing, based on the assumption that no researcher has been able to measure more than one kind of reading ability. The concept of "lexiles" is introduced as a measure for readability which uses word familiarity and sentence length. The Lexile formula is explained, and processes for using the Lexile concept and applying it to reading instruction are discussed at length. The paper ends by relating Lexile scores to jobs, amount of education, and income. Contains 44 references, 2 notes, and 5 tables and 20 figures of data. (EF)