Just back from those magnificent Ethiopian highlands I can recommend the Lonely Planet Guide Book 'Ethiopia and Eritrea' as the most useful guide to Ethiopia by far!
I had quite a couple of guide books in my backpack - some of them more detailed in history, some of them far better in matters of the Ethiopian Orthodox Church. But still: as an overall guide book this Lonely Planet is simply outstanding.
Rich in details, broad in travel suggestions, just in its judgements and very sympathetic to the people and their culture.
Of course, you will find blank spots and outdated information.
Still - this is the book to rely on!