Tales of the Dying Earth By Jack Vance
Weird and wonderful set of stories, especialy Cugels Saga, that have influenced everyone from George RR Martin to Gary Gygax. A lot of original Dungeons and Dragons came from this book and it could be credited with as much influence as Lord of the Rings on modern fantasy literature and games. Quirky, witty and often very funny, a great read.
The Swords series by Fritz Leiber.
Thi set of tales concern Fafhrd the barbarian and his rogue companion The Gray Mouser. written in the 40s and 50s like The Dying Earth series, they are a rip roaring, quirky set of tales that established, like trhe Dying Earth, a lot of the conventions of modern fantasy. Pratchett has parodied a lot of bits of this set, the beginning of Colour of magic was lifted right out of the Swords Series - when Bravd the Hublander and the Weasel watch AnkhMorpork burning from fires they have just set it parodies Fafhrd and The Gray Mouser watching Lankhmar burn in similar circumstances. And the names of both cities aren't a million miles from each other either.
2 good sets of reading and you will smile at how much the bread and butter of modern fantasy owe to these seminal works.