This bibliography is a working draft, insofar as no authoritative bibliography of observers (whether as formal periti or informally invited scholars) exists at this time.
An additional difficulty is conclusively to identify particular writers as Protestant Christians. For example, newspaper and other media reporters might have been formally Protestant (whether closely affiliated or distantly related), but did not identify their writings or observations as relevant to or influenced by any Protestant point of view. As a result, this bibliography lists entries whose authors in some sense identified themselves as Protestants, or representing the viewpoints of Protestant churches and organizations.
The appended, additional bibliography lists discussions of and responses to Karl Barth’s response to the Second Vatican Council, taken as a whole, in 1965. Health prevent Barth from attending the sessions as a peritus or unofficial observer, but his reflections published following his meeting with Pope Paul VI occasioned discussion in theological circles of the time. They are there most relevant to shaping Protestant historical memories and reflections upon the work of the Council, and the ongoing issues that it identified.
Ferriby, Peter Gavin, "Protestant Observers of Vatican II: A Bibliography" (2009). Presidential Seminar on the Catholic Intellectual Tradition. 21.