Mike Piper, Investing Made Simple: Index Fund Investing and ETF Investing Explained in 100 Pages or Less, Lightning Source Inc., 2009
Simple to read, yet informative. This is one of the books that require fewer pages and lengths to understand a topic, yet it covers pretty much everything you need to know about investing (and specifically index funds). The book gives the reader a whole summary of investments and index funds, with easy-to-understand language and very relatable examples for easy understanding.