Okay, thanks. But 'mushroomed' isn't formal, in fact, in this case it's probably sort of humorous, since the thing grew out of what was evidently just a short report or list. That is, 52 pages probably really isn't enough for a literal full-sized volume, so 'book' may be tongue-in-cheek, unless it's shorter things like recipes or knitting patterns or something.
There's really nothing a little more vivid, no image of growing at all? 'Umfangreicher geworden' is just so colorless and official-sounding ...
And, again, do the suggestions in #4 need adding to LEO, to cover cases like this? Because this sense of 'mushroomed' is really probably the most common one these days. Another typical collocation is with a job, task, duty, chore, etc., e.g.,
I thought being on the committee would just involve attending a few meetings, but the job has mushroomed into fundraising for a whole new building.