Probably one of the multi-book anthologies I’ve bought on Kindle. Some of them have included 20 books. However, I think that’s 20 books in one volume, not a single book.
On that basis, the longest single title I’ve read is probably War and Peace. I checked a couple of “longest book” lists online, and War and Peace is apparently 1225 pages long, or 587,000 words.
The longest Christian book I can think of reading is Glastonbury by Donna Fletcher Crowe, which is 1134 pages long. That may or may not be longer than War and Peace: bigger fonts make for longer books regardless of word count.