Here are the books I have used to gather information about the churches and the region.

Les églises fortifiées de la Thiérache - Picardie, Aisne

Hervé Milon, Yves-Marie Lucot / Edition Chamina / ISBN 978-2844661104
This short book gives a good overview and is the best choice for occasional visitors of these churches

Les églises fortifiées de la Thiérache

Jean-Paul Meuret / Societé Archeologique de Vervins / now only available in second-hand versions
This is the standard work on the fortified churches of the Thiérache, to which all other authors refer

Les églises fortifiées de Thiérache

Text: Gilles Grandpierre / Photos: Jean-Marie Lecomte / Editions Noires Terres / ISBN 978-2-900446-16-4
The latest book with plenty of photos and detailed description, also about the history of the region

Trésors de Picardie - Les Églises fortifiées de la Thiérache

Robert Poujol / Albert Lemasson (Illustrateur) / ISBN 2-7428-0054-9 / Rassort - Lorisse
Another book with short descriptions and some nice black-and-white illustrations of the churches