For general safari planning, you can't go wrong with Fodor's THE COMPLETE AFRICAN SAFARI PLANNER which covers Kenya, Tanzania, South Africa, Botswana, Namibia and Victoria Falls. For each country, the guide covers the gateway city, the must-see parks, the if-you-have-time parks and beach escapes. It also gives you great tips on the ins and outs of safaris including what to expect, what to wear, what to bring and more. It was thanks to this guide that we decided we must go to Ngorogoro Crater and Lamu. Though I didn't end up using any of the eating or sleeping listings, I enjoyed the wealth of cultural information presented throughout. Definitely a keeper. It may even inspire me to go on more safaris!
As for day-to-day guides, we used the Lonely Planet's EAST AFRICA as well as country guides from Footprint covering Kenya and Tanzania. All three offered a good mix of both affordable listings and more upscale locales. I found the Lonely Planet to be more reliable in general, although current phone numbers can be a problem in both publisher's books. Your best bet is to check the listing's website for the most current contact details. We had a devil of a time finding the number for Robert's Camp in Baringo, though it didn't turn out to be a problem since we ended up being the only guests.
Footprint's KENYA HANDBOOK is current as of March 2009 but some of the information didn't get double-checked when they printed the new edition. For example, the guide claims Diamond Village (where we stayed) has pit latrines, but they've had en-suite flush toliets since 2005. Oops! What I do really like about the Footprint guides are the hard covers and the excellent maps.
The book I used the most during my trip was Lonely Planet's WATCHING WILDLIFE EAST AFRICA. Not only is it full of excellent color pictures and detailed information about species you are likely to encounter, it also has a whole section covering each national park in East Africa and what wildlife you may find there (and in what areas specifically). Whenever our guide would point out a new bird (for example), I would pull out the book and read all about it. The only think I was really missing was a checklist so I could keep track of what I'd seen and what I still needed to track down.
What guidebooks do you like to use when you travel?