There are many good guidebooks on Tibet. If you plan on being in in Lhasa and the Tibet Autonomous Region (TAR), Lonely Planet's Tibet guidebook is the best option. It is well written, easy to understand and has the most reliable information for this area of Tibet. In addition to the TAR, Lonely Planet's Tibet also covers most of Kham except for the areas found in Qinghai and Yunnan. It does not cover any of Amdo.
The best guidebook for the Amdo and Kham regions of Tibet is the Footprints Tibet Handbook written by Tibet scholar Gyurme Dorje. Though it is big, it covers all areas of Tibet including every county on the Tibetan Plateau. The only problem with it is that it often uses Tibetan place names that have either changed or are only known by the local villagers which can cause problems trying to reach really remote areas. Another good guidebook choice for Amdo and Kham is Lonely Planet's China Guidebook. It covers the Kham areas of western Sichuan, northwest Yunnan and southern Qinghai as well as the Amdo areas found in Qinghai, northern Sichuan and southwest Gansu. Though it needs to be updated badly as it is about 10 years old, Mapping of the Tibetan World contains several hundred maps which are still very useful. Mapping of the Tibetan World covers all of the Tibetan regions found in China as well as the Tibetan inhabited areas found in India, Nepal and Bhutan.
If you plan to go trekking anywhere in Tibet, you need to get a copy of Gary McCue's excellent Trekking Guidebook. This book contains all of the major treks in Tibet, including those in Amdo and Kham. Mr. McCue has been trekking around Tibet well over 25 years and has put together the BEST guidebook for trekking the region.
If you have any other questions about guidebooks for Tibet, feel free to contact me at firstname.lastname@example.org