These are some of the books I have purchased and found useful:
The ESL Teacher's Book of Lists ISBN: 0-7879-6738-6
English for Foreign Language Speakers: The Easy Way ISBN: 978-0-7641-3736-5
McGraw-Hill's Essential ESL Grammar ISBN: 978-0-07-149642-1
Practice makes Perfest: English Grammar for ESL Learners ISBN: 0-07-144132-8
Practice Makes Perfect: English Vocabulary for Beginning ESL Learners ISBN 978-0-07-146085-9
Specificallly for teaching English to Hispanics:
Domine lo Básico Inglés : Master the Basics of English for Spanish Speakers ISBN: 978-0-7641-2192-0
Manual completo de los verbos en Inglés ISBN: 978-0-07-144496-5
The Firefly Spanish/English Visual Dictionary ISBN: 978-1-55297-951-8 (note: This book contains graphic depictions of the human body so may not be appropriate for younger students to browse)
Inglés Para Todos Los Días ISBN: 978-0-7641-2121-0
Inglés para Latinos ISBN: 978-0-7641-1990-3
Madrigal's Magic Key to Spanish ISBN: 978-0-3854-1095-3 (just reverse the rules for learning Spanish to learning English)
Harper's Spanish and English pocket Dictionary ISBN: 978-0-07-145669-2 (one of 2 dictionaries I recommend for students)
VOX Everyday Spanish and English Dictionary ISBN: 978-0-07-145277-9 (the other dictionary I recommend)