I have drawn up a list of essential resources:
Osprey's Men at Arms no. 95 The Boxer Rebellion by Lynn E. Bodin. A great general reference and the painting guide.
Osprey Campaign Series no. 85, Peking 1900 by Peter Harrington. Good photos and illustrations, bird's eye view of Peking legation, and maps.
The Boxer Rebellion by Paul H. Clements. Addresses causes, politics, and may give some insight into motivations. No photos or illustrations.
The Siege at Peking by Peter Flemming. Offers some useful details with respect to modelling and general details of the relief expeditions and battles. Few maps, no photos.
James Ricalton's Photographs of China During the Boxer Rebellion by Christopher J. Lucas. Only a few photos were specifically of the legations.
The Colonial Wars Sourcebook by Philip J. Haythornethwaite. A small section is dedicated to the Boxer Rebellion, it offers an overview of the Rebellion. B&W photos throughout.
The Fists of Righteous Harmony by Henry Keown Boyd. Similar to Flemmings book above. Many good photos and maps.
The Boxer Rebellion by Diana Preston . A well-written account of the Boxer Rebellion but as seen through the eyes of the Western participants only.. Some b&w photos and maps.
Miniature Wargames magazine Nos. 65,66,67,68 (Oct '88 thru Jan'89) have a four part article. It covers uniforms, history, maps, etc.
Practical Wargamer magazine Autumn '88 has an article on the British Legation with birdseye view.
Wargames Illustrated no. 27 contains an article called "55 Minutes at Peking", which inspired me for the period. Contains pictures and rules.