The following guidebooks are the best guidebooks that will easily fit in your pocket.
Pocket-sized guidebooks are a little thin to get true insight into Spain, but the two books below each have their advantages and may be perfect for your trip to Spain.
Berlitz are the undisputed experts in pocket travel guides, making pocket guides to just about every major city in Spain. It is a little brief, but is the perfect guidebook for those that don't like guidebooks! Good for those on whistle-stop tours of Europe who know they won't spend that long in Spain. If you know where you'll be going and need a little more detail, go for the AA Spiral Guide (below) instead.
This spiral-bound guide to Spain would be a little big to put in your pocket at first, but the pages are easy to remove, meaning you can easily shrink it down to a manageable size. Not ideal for those who don't yet know where they'll be going (in which case the above Berlitz guide would be better).