Pretty much the latter. It's not like FO3 where there's kind of a set amount placed in specific areas - it's more of just another type of loot. (And yeah - after a certain level they stop adding to your skills.) They can be pretty useful, but I wouldn't recommend building a character around it or anything.
There's even a couple shops (and I think a Librarian) that will sell you large amounts (and restock every couple of days) so you can up some of your skills pretty easily going that route.
But it's not so much like they're scattered around for you find - I wouldn't recommend setting yourself a goal of finding all of them (since you really can't, as far as I know.)
Yes in Fallout 1 there's a librarian in The Hub who restocks Big Book of Science, Dean's Electronics, Medecine, Scout's Handbook and Guns n' Bullets.
They are useful when you have money to waste on and you are very low on those skills.. it's especially useful for the First Aid skill.
As for Fallout 2... I believe there's one in Vault City.