Practice Makes Perfect: Spanish Verb Tenses This extremely popular book helps you master the Spanish verb tenses. This would be an excellent book to use after completing Getting Started with Spanish.
Easy Spanish Phrase Book This inexpensive book (only $2 on Amazon) contains hundreds of useful phrases. And underneath each phrase it shows you how to pronounce it, too. Very useful.
1001 Most Useful Spanish Words This book is just as cheap as the last one I mentioned (2 bucks). Contains useful words to memorize and use.
Free Resources from Google Books
History of Spanish Literature This book traces the history of Spanish literature from around the 12th century to the 18th century.
Easy Spanish Plays These plays allow you to study conversational Spanish.
A First Spanish Reader This book has easy Spanish stories you can practice with.
Spanish Tales for Beginners This book has more Spanish stories you can practice with.
An Elementary Spanish Reader Still more easy Spanish stories to read.
Easy Spanish Reader Even more stories.
A Progressive Spanish Reader Another reader with more stories.