Books on Teaching English as a Foreign Language in Spain.
The English teaching bible, usually essential advanced reading for EFL teachers before they begin their TEFL course.
Excellent Grammar reference book, with exercises and great grammar explanations for both the learner and unsure teacher! Every language school in Spain has at least one copy of this book.
Advanced grammar book, more for the teacher than the student. Never be baffled again by a student's question!
Paperback edition of the above book. Won't last as long, but might be easier to transport around.
You'll no doubt have to teach business classes in Spain (especially in Madrid and Barcelona) so this book should come in handy.
One-to-one classes are popular in Spain. This book is especially useful if you plan on teaching private classes (where you can charge more than you would earn in a language academy).