The first Moon handbook I encountered was the classic South Pacific Handbook, by David Stanley. Then I acquired another classic, J. D. Bisignani's Hawaii Handbook. They were just what I wanted in a travel guide - compact, yet crammed with detailed information about places, their history, culture, environment, everything. Plus necessary information about travel practicalities, and good sound advice.
The Moon Handbook series is extensive, and growing. The trend is to break up the massive original works (such as South Pacific and Hawaii) into smaller units. Bisignani died a couple of years ago, but his work has been preserved in the continuously updated editions by Robert Nilson. Stanley is still out there, a world class traveller, moving anonymously around the Pacific. A whole crowd of others have been recruited to cover every one of the western states.
The handbooks all follow a common plan. There is first a lengthy introduction, covering the geography, history, and culture. Then general information on shopping, museums, how to get there, getting around, activities, and what to take. Next comes a detailed survey, area by area, each with sections on sights, accomodations, food and shopping.
The quality varies a bit, but the range is from good to excellent. The maps tend to be small and schematic, but new editions have better maps, including color (a couple of my former students are working on this).
I always take a Moon Handbook when I travel. I have particularly enjoyed Stanley's South Pacific, Bisignani's Hawaii, and Hempstead and King's British Columbia.