I have never had such enjoyment reading a book this far out of my league!
I will give a strong warning that this is not for a casual reader; if you do not have a deep and intricate knowledge of 19th century German politics (and European politics, religion, society, and culture as a whole), you will very quickly be lost. For a more approachable work on Windthorst, I recommend "The Lived the Faith: Great Lay Leaders of Modern Times," by Thomas Patrick Neill (1951).
I wanted to learn more about Windthorst after reading a biography of Bismarck, and this was one of the few works on Windthorst I could find. I'll readily admit that I didn't understand much of what I read, given the book's depth and complexity. However, the author is a joy to read, and I have new and immense love for Windthorst. As one of the best politicians and most prominent Catholics of his generation, but plagued by enemies, losses, and humiliations, he should be a great consolation to current Catholics. Given what a titan he was, I am appalled that I never heard of him. Despite how challenging this book was, I can't wait to reread it.