My inventory has about 50 books and other writings in it. I never had even close to that number of books in Morrowind in my inventory.
I'm pretty sure that several of the booksellers in Morrowind have more than that in their inventory, and they only carry a small selection (Jobash in Vivec has a slightly larger inventory of "rare" books). Counting just the "series" books, like Vivec's lessons, the multi-part History of the Empire, various travel guides, and the numerous multi-part adventures like those about Argonia and the "Barenziah" series (plus the "Real Barenziah" versions), that's pretty close to 50-100 right there, if not more. The huge numbers of individual topics have got to top 100 more at minimum.
I recall perusing the books in the various Mages Guilds in Oblivion, and it was mostly the same dozen books over and over. The booksellers had more, but only 1/4 the selection that they had in MW, probably due to the annoyance of scrolling through them all on a console.
A number of books made their way from DF to MW, although several had to be edited for the "mass market" audience (mods restored a few to their original form). Many of the books in MW reappeared in OB, but most of them didn't.