I'm not OP, but I also have thousands of books (a little over 4100 at last count), so I guess my answers might be of interest.
1. I'm about 50 years old. (The furthest back my records go is 2004, when I was about 35 and the count was about 2300.)
2. Of the 4100 books in my catalogue (it's a CSV file, very high-tech), 1233 are fiction. Rough counts for some non-fiction subject areas: science 520, mathematics 420, philosophy 250, computing 220. Others with substantial numbers: religion 270 (I'm not religious, but I used to be and my wife still is), humour 240, history 210, children's books 180 (may be wrong; we don't always bother to record these and sometimes we get rid of them since children grow), reference works 110 (dictionaries, encyclopaedias, etc.), music 100, puzzles 100, poetry 100, language 100, books of essays 100, cookery 100 (these are mostly actual cookery books, which maybe don't really belong in the same list), biography 90, literature 90 (meaning literary criticism, books about books, that sort of thing; actual works of literature are mostly under "fiction"), politics 80, autobiography 75 ("biography" earlier excludes these), games 65 (this is things like chess books, not books that are somehow also games), education 65.
3. I think about 10% of the books are unread at any given time.
1. I'm about 50 years old. (The furthest back my records go is 2004, when I was about 35 and the count was about 2300.)
2. Of the 4100 books in my catalogue (it's a CSV file, very high-tech), 1233 are fiction. Rough counts for some non-fiction subject areas: science 520, mathematics 420, philosophy 250, computing 220. Others with substantial numbers: religion 270 (I'm not religious, but I used to be and my wife still is), humour 240, history 210, children's books 180 (may be wrong; we don't always bother to record these and sometimes we get rid of them since children grow), reference works 110 (dictionaries, encyclopaedias, etc.), music 100, puzzles 100, poetry 100, language 100, books of essays 100, cookery 100 (these are mostly actual cookery books, which maybe don't really belong in the same list), biography 90, literature 90 (meaning literary criticism, books about books, that sort of thing; actual works of literature are mostly under "fiction"), politics 80, autobiography 75 ("biography" earlier excludes these), games 65 (this is things like chess books, not books that are somehow also games), education 65.
3. I think about 10% of the books are unread at any given time.