Last of the Pirates: The Search for Bob Denard by Samantha Weinberg tells the extraordinary story of the French mercenary who staged multiple coups in the Comoros and essentially ran the country for stretches of the 1970s and 1980s. It reads like a thriller but it is all true, and it explains a great deal about why the Comoros is the way it is politically. Essential background reading.
Lonely Planet Madagascar and Comoros by Paul Greenway and Deanna Swaney is dated now but remains one of the only English-language guidebooks that covers the Comoros in any detail. The practical information is largely obsolete, but the background sections on history and culture still hold up. Check for newer editions before your trip.
The Comoros Islands: Struggle Against Dependency in the Indian Ocean by Malyn Newitt is the most comprehensive English-language history of the archipelago. It is academic but readable, covering everything from the early Arab settlers through French colonization to independence and the turbulent decades that followed. Worth the effort if you want to understand the political complexity.
Amphibians and Reptiles of Madagascar, the Mascarenes, the Seychelles, and the Comoros Islands by Friedrich-Wilhelm Henkel and Wolfgang Schmidt is a specialist book but worth knowing about if you are interested in the unique wildlife of the Indian Ocean islands. The Comoros has several endemic species, and this is one of the few references that covers them.