Read the UESP. It holds about 500ish books
I counted them myself when I was filling the shelves up 308 books.
First floor has 5 bookshelves holding 220 books
8 shelves holding 11 books
4 shelves holding 11 books
2 shelves holding 11 books
4 shelves holding 11 books
2 shelves holding 11 books
Ground floor has 3 bookshelves holding 88 books
2 shelves holding 11 books
4 shelves holding 11 books
2 shelves holding 11 books
Now you count them yourself if you do not believe us.
There is also a chest, satchel, strong box and a couple of sacks for extra storage in the tower, I lost interest in it pretty quick.
Edit
Corrected for correct floor description