For me, hands down the best 1 book RPG is DragonQuest, 2nd Edition. Great fantasy game with lots of flexibility, a creative method of character creation, magic that is interesting, and dragons that are creatures to be feared and avoided. Lots of fun, I played it for years.
Oh, and as for most RPGs being one book, Traveller started as 3 books; GURPS is now two volumes of one large book; Marvel SuperHeroes was 2 booklets; and there are other examples I am missing because it is early on a Monday.