assuming 1TB drive
average "epub book" like a best seller on the NY listing is about 1.5MB each
average "PDF book" like a best seller on the NY listing is about 50 MB each
average "epub illustrated book" is about 50MB each
average audiobook can run easily 500MB each
Based on a WD Black 1TB HDD I own 1TB drives is 931GB or 1,000,202,006,528 Bytes of information
and understanding that 1.5MB = 1,572,864 Bytes, 50MB = 52,428,800 Bytes and 500MB = 524,288,000 Bytes
you can calculate it can hold (if only holding this information and nothing else (i.e. a second storage drive for this purpose alone)
1,000,202,006,528 / 1,572,864 = 635, 911 epub Books or 1.5MB size
1,000,202,006,528 / 52,428,800 = 19,077 of 50MB Size PDF/Illustrated Books
1,000,202,006,528 / 524,288,000 = 1,907 of 500MB audiobooks
hope that was detailed enough for you