Lonely planet book itself: The book was ok for a general overview of each area, but I found that the listings were not very comprehensive and the descriptions not particularly helpful.
* Xela has at least 40-50 language schools, but the guide lists only five, and doesn't at least acknowledge that it is not at all a comprehensive list. I found my school by word of mouth. It makes no mention of Adrenalina Tours, a Guatemalan-owned national operation that actually has a fairly strong set of offerings, and has English-speaking staff.
* Antigua section did not at all include a number of restaurants that I passed, and did not describe a number of the buildings, nor did it include the hotel (Hotel del Carmen) where I stayed - and this was a fairly sizeable, mid-range hotel.
* The Panajachel map does not include many of the narrow side streets that cannot accomodate cars but do accomodate pedestrians and scooters.
I had the kindle version, which was also problematic: the index was not very good, and there was no direct way to get to the table of contents. I would suggest, too, that it might be better to walk around with a paper book - it's not very practical to walk around with an expensive e-reader or iPad.