Not only does the size of the book lend itself to being on the trail (easily stuffs into a camelpack or daypack), but it does a good job of explaining some basic repairs or problems you may encounter during a ride. Such things covered include: brake pads rubbing against the rim, unable to unclip from your pedal, a jammed chain, loose hub, or simply not having tire levers. A description is given of the problem, how to diagnose the problem, and what steps you can take to repair it yourself (or at least get back to the trialhead).
This book also provides diagrams, labeling components. This is beneficial to new riders who may not be certain of what is a cog, derailleur, or hub for instance.
While there may be other, more detailed repair books out there, this is a great book that you can easily take with you as you are first learning how to fix some common problems yourself. Definitely a good pocket guide.