The free Scottish Mountain Biking Guide is a great compact introduction to riding north of the English border.
Helpful safety and travel tips, trail grading information and even some mountain biking lingo, deciphered by World Cup downhill champion Steve Peat, who also wrote the foreword, make this a good introductory guide to the sport.
Particularly helpful for the more experienced rider are the trial guides towards the back of the book. From 7stanes to FortWilliam, each one has a quick facts column with the types of routes you can find there, a list of bike hire shops in the area and accompanying pictures.
This guide makes you want to find a way up to the Highlands to test the trails on offer. You can either view the guide online or request a copy through the VisitScotland site.