One of Lonely Planet's best efforts, this guidebook is comprehensive and accurate, with good photos and lots of helpful information. It covers Kenya, Tanzania, Uganda, Rwanda and Burundi, plus a small segment of the DR Congo.
There is the usual information about sights and activities, full of addresses, prices, and everything you need to know. The full colour wildlife guide is handy (especially for distinguishing between antelopes) and there is a whole section devoted to safaris and gorilla trekking. The history sections for each country and for the region in general are easy to read and informative.
The 'directory' section includes useful information on everything under the sun, plus there are chapters devoted to health, transport, and language. The introductory information at the start of the book covers culture, art, food, history, etc. The maps are clear and reasonably easy to read.
I looked through both this guide and specific country guides for those I was visiting in the region, and decided this one covered pretty much everything you need but in one volume, making it a much better bet for travelling with.