The "Guide Book of United States Coins, 21st Revised Edition 1968" by R. S. Yeoman is a comprehensive resource for collectors and enthusiasts of United States coins. Published by Whitman, this guide provides detailed information on various types of coins from the U.S., making it a valuable tool for those looking to expand their knowledge and collection. With revisions and updates made in 1968, this edition offers the latest information on coins from that time period, making it a must-have for collectors and numismatists.