The Council of Constance edited by Louise Ropes Loomis (1961) — a primary source anthology covering the council sessions from 1414 to 1418. Dense but not academic: the records of the debates, the condemnation of Jan Hus, and the election of Martin V are all here. Reading it before visiting the Konzilgebäude changes the building from a pretty waterfront hall into something that feels genuinely weighted with consequence.
The World of Yesterday by Stefan Zweig (1942) — Zweig's memoir of a vanished Central European world, written in exile just before his death. His world was Vienna and the German-speaking bourgeoisie of the late Habsburg era, and while Konstanz is not his subject, the cultural landscape he describes — educated, cosmopolitan, deeply European — is exactly the world that still echoes in the old towns of Baden-Württemberg and the Swiss border region. One of the great memoirs of the twentieth century.
Jan Hus: His Life, Teachings and Death, After Five Hundred Years by David Schaff (1915) — a biography of the Czech reformer burned at the Council of Constance in 1415. Hus came to Konstanz under a guarantee of safe conduct and was executed anyway; his death sparked the Hussite Wars and is one of the episodes that made the Reformation inevitable a century later. Schaff is thorough and the account of the trial is gripping.
Bodensee: A Sailing Guide by various — there are several practical sailing guides to Lake Constance in German, and one or two in English, that read as travel books as much as navigation aids. The lake is large enough to have its own weather patterns, its own maritime culture, and its own ports — reading one gives a sense of Bodensee as a world in itself rather than a backdrop.