Osprey's Men at Arms no. 95 The Boxer Rebellion by Lynn E. Bodin. A great general reference and the best painting guide that I have seen. There appears to be a couple errors (I'll detail those when time permits), but still worth its weight in gold, or at least figures.
Osprey Campaign Series no. 85, Peking 1900 by Peter Harrington. Very inspirational, good photos and illustrations, great bird's eye view of Peking legation, and maps, helps somewhat with putting scenarios together (a real problem with the Boxer Rebellion).
The Boxer Rebellion by Paul H. Clements. Very dry, doesn't offer much for the gamer on the tabletop, though addresses causes, politics, and may give some insight into motivations. No photos or illustrations.
The Siege at Peking by Peter Flemming. I thought reasonably entertaining, though with some dated information. Offers some useful details with respect to modeling and general details of the relief expeditions and battles. Not enough to put any detailed scenarios together. Few maps, no photos.
James Ricalton's Photographs of China During the Boxer Rebellion by Christopher J. Lucas. Not really a gamers book. Lots of interesting photos, but not that many specifically related to the war. It was generally helpful to modeling buildings, but only a few photos were specifically of the legations. Kind of pricy relative to its gaming value, but very interesting if you're into history and culture beyond the war or wargame.
The Colonial Wars Sourcebook by Philip J. Haythornethwaite. Though only a small section is dedicated to the Boxer Rebellion, it is a wonderful, concise book offering an overview of the Rebellion, and most other colonial wars, including those in China having relation to the Boxer conflict. Many other entries can be of some help with respect to putting the game on the table. B&W photos throughout.
The Fists of Righteous Harmony by Henry Keown Boyd. Similar to Flemmings book above, but with some updates and corrections, generally better though; probably the best boxer history that I have read . Many good photos and the maps are helpful (though not extremely detailed) in putting together scenarios.
The Boxer Rebellion by Diana Preston . I've only read a small portion of the book, but found what I read to be entertaining and offered a few details that I had not picked up before. Some b&w photos and maps. I 'll write more when I finish it.
Miniature Wargames magazine Nos. 65,66,67,68 (Oct '88 thru Jan'89) have a four part article that can be very helpful. It covers uniforms, history, maps, etc.
Practical Wargamer magazine Autumn '88 has a very nice article on the British Legation with birdseye views, a great help for modeling.
Wargames Illustrated no. 27 contains an articel called "55 Minutes at Peking", a rather light hearted, but very fun set of convention rules, also with a few inspirational photos of a Peking model.