I did a 100% run for Oblivion years ago - unfortunately the book stat doesn't record how many individual books you've actually opened (it records all books you've opened, so loads of identicals), so I'm not sure if I came close to reading them all. But I read a lot!
As
@Artoriarius put it, it was the diverse nature of all the books, 'manuals', histories, 'histories'
, recipe books, tall tales, diaries etc. that was wonderful, and it did more to flesh out the world of Tamriel for me than a fan fic or tie-in novel. Also more than the 'revolutionary' NPC system. (Which tended to feel half-broken!)
Near the end of the run I was so totally OP against the monsters and bad guys, and so rich with loot and gold, that I was completing the last of the dungeons and quests just to find books I hadn't read, to add to my library chest.
Ah good times. I left my 100% character at the end, when everything had been done, on top of the lighthouse at Anvil, forever watching a beautiful sunset over the Western sea.