I have used the Lonely Planet Guide for two visits to Ethiopia. I haven't been to Eritrea so I can't comment on the guide for that country.
The Lonely Planet series, in general, is short on describing luxury accomodations, dining, and chichi places and long on local history, customs, and out of the way places. Described in the guide, for example, are the dubious pleasures of chewing "chat," the lovely Ethiopian coffee ceremony, and the possibilities for trekking in several Ethiopian national parks.
Ethiopia is one of most contradictory and complex countries in the world. It it one of the oldest Christian countries, claims to house the Ark of the Covenant (remember "Raiders of the Lost Ark") in a church in Aksum, boasts "Lucy" the bones of one of the oldest ancestors of human beings, and has some of Africa's most spectacular scenery, the Blue Nile falls for example. Ethiopians have a strong, confident ancient culture and perhaps the most distinctive cuisine in the world. Steak tartare fans will like kifto. Cold beer and hot pasta (reflecting the brief Italian colonization of Ethiopia) can be found most places for less adventurous eaters.
One tip from me that may not be in the guide. Ethiopians for some reason are reluctant to accept 1996 US $100 bills and any bill that is torn or worn.
Ethiopia is a country well worth visiting. This guidebook will tell you all you need to know.