I found the Lonely Planet guide to be an excellent resource, especially when used with the Baedeker Scotland guide. LP offers truly helpful information, such as where to find maps, how to get back your VAT, weather and insect warnings... as well as brief but very descriptive chapters by region, including the oft-neglected Highlands.
Suggested itineraries are included, and the descriptions of sights, towns and historical destinations is wonderful.
While the guide is short on maps and history -- this is where the Baedeker guide does an excellent job -- it is full of useful and up-to-date information on travelling Scotland. I found my copy to be incredibly resourceful.