Brandon Simpson, co-author of Getting Started with French, says that his is his favorite follow-up book for beginning Spanish students (paid link).
Practice Makes Perfect: Spanish Verb Tenses This extremely popular book helps you master the Spanish verb tenses (paid link).
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.
Live Spanish Classes Online
Simply Teaching Spanish Señora Clara Stacko has been using Getting Started With Spanish to teach Spanish to homeschool students for many years. Now, this native Spanish-speaking teacher is offering online classes to homeschool students in grades 5-12. You can register for her classes by visiting her website.