There are over twenty volumes in St John's Library which once belonged to Julius Echter von Mespelbrunn, Prince-Bishop of Wurzburg. All except one of these bear similar alum-tawed, white pigskin bindings over wooden boards, decorated with blind-stamped panels and rolls, of a type that was popular in Germany from 1550 to 1640. They all have Mespelbrunn's arms stamped and then painted in red and black on the front cover, and several also have a gilt mark of ownership pressed into the fore-edge of the textblock, together with the title of the particular volume. The armorial panels are usually similar, but there is a volume with a circular variant. The one book that does not have this distinctive binding is a gold-tooled vellum binding, with a painted central ornament portraying Mespelbrunn's arms. Virtually all of these volumes were donated by Richard Holdsworth, a fellow of the College, although one was given by Peter Gunning, and another two have an inscription with the name James Tabor. They were part of the spoils of a Swedish army which looted Wurzburg and Mespelbrunn's library during the Thirty Years War. A sale of duplicates was later held in Sweden which Holdsworth appears to have attended, later distributing volumes among the various Cambridge libraries. The items that have ended up at St John's are mainly a mix of Catholic theology, law, and medicine, including a work on the analysis of urine.

Mespelbrunn became Prince-Bishop of Wurzburg at the age of 28, and proved to be an energetic and able ruler, founding a university, building churches, reforming the administration of justice, and establishing primary schools. He also promoted the ministry of the Jesuits in his province, banned Lutheran preachers and became a major figure in the German Counter-Reformation.