[quote="i_am_a_Catholic, post:1, topic:292936"]
I was wondering if there were any Catholic Bishops who joined the cause of the reformation when it took place?
Some. In Germany the only one I know about Hermann von Wied of Cologne (I think a later archbishop of Cologne may also have tried to go Protestant, but I don't know much about it). And his case is complex--he's one of those of whom I think it can genuinely be said that he never intended to leave the Church. He attempted to work with both "Protestant" (Martin Bucer) and "Catholic" (Johann Gropper) reformers in order to reform the local church in Cologne. But the effort broke down--Gropper denounced the archbishop's invitation to Bucer to be his court preacher, and eventually the Pope deposed and excommunicated von Wied.
In Scandinavia and England, bishops did become Protestants, although even there in a number of cases the Protestant bishops were newly consecrated and not the same as their Catholic predecessors. I know more about England than Scandinavia--there all the bishops but one resigned in 1559 rather than take an oath to Elizabeth. Earlier, under Henry and Edward, the story was different, though. Certainly most of the bishops went along with Henry's break with Rome (if you count that as "Protestant"), and after that break a number of people became bishops who had Protestant sympathies--most notably Thomas Cranmer.
Also wondering, is there any good book to read about the historical facts (not necessarily the theological or doctrinal) around the reformation? As in who said what, who did what, who joined who etc?
Well, the two tend to be connected. A few suggestions:
Diarmaid MacCulloch's The Reformation is comprehensive and very readable. He's an agnostic ex-Anglican with fairly pro-Anglican sympathies (he's also the author of the best biography of Thomas Cranmer), but I think most folks would agree that he's quite fair.
Euan Cameron's The European Reformation is one of the classic surveys. I read it in grad school and thought it was very fair-minded, though Cameron is himself a liberal Protestant (Anglican I presume).
Carter Lindberg's The European Reformations is another well-respected survey. Lindberg is Lutheran.
For Germany in particular, Thomas Brady's German Histories in the Age of the Reformations is excellent. I'm currently reading it, and find it extremely helpful. He is a master of sixteenth-century political history and is able to provide succinct overviews of important issues that are extremely fair and backed by decades of careful research. For instance, the chapter I've just been reading on heretics, Jews, and witches ought to be read by everyone on this forum who gets involved in debates about persecution in the early modern era.
Another author you should definitely check out is Brad Gregory. Gregory is a Catholic scholar who came to prominence for his book Salvation at Stake, *dealing with sixteenth-century martyrdom. More recently, he's written a book called *Unintended Consequences arguing that the Reformation is the cause of the secularization of the modern West. I have not read it yet, but it's been causing quite a stir.