Mouseferatu
Hero
How often do you think pcs will find these items? Do you really expect a pc to find multiple books that increase the upper bounds of the same stat? What's the rarity of those books again?
Any given tome? 1-in-100 on the actual table, and the odds of getting to that table are, depending on level, either 2, 8, or 10-in-100.
So 1% of 10%. At most. Per book. And it takes six of the same sort of book (or four, if you're talking about a barbarian's Str or Con at 20th level) to break a stat of 30.
In other words, really not gonna happen unless the DM makes it happen.