Votan,
I actually should have specified *brand new books*, instead of just *books*. Because, last time I checked (just the other day, in fact

) the second hand RPG book market was HUGE. Seriously, you can get just about anything for anything, and
usually at a reasonable cost. If you don't mind second hand stuff, this is, of course.
So really, even if the expected personal library for say, D&D 3.5 happens to be
20 books 
then if someone needed to, they could probably pick up
at least the majority of those, and quite likely
all of them, with ease.
Yes, there are some crazily collectible items out there on
eBay, or
Amazon, or sometimes a place like
Noble Knight, perhaps. But for most games and supplements in existence, even if they're quite old, things aren't even approaching that kind of wackiness.
So, if getting (or, in some cases, printing and binding) books isn't a problem. . . what is, I sometimes wonder. But then, I suppose, it's just part of the culture or whatever. Eh, like I said, never mind.