I just finished reading Lin Carter's World's End series (Warrior of World's End, The Enchantress of World's End, The Immortal of World's End, The Barbarian of World's End, and The Pirate of World's End). It's also known as his Gondwane series as that's the name of the last continent on Earth in the year 700 million AD, when the stories are set.
The series is quite imaginative and seems to be a great influence on Gamma World. There are Hoppers, which are like a cross of goblins and rabbits, mobile plants, mobile vegetables, mobile cities, mobile castles, mobile mountain ranges, and mobile islands! There are Death Dwarfs, green-skinned dwarves that eat poison, gyraphonts, lobster-ghouls that flit between dimensions, and strange little men of the hills, which are a mineral lifeform - sort of an earth elemental. There are also tigermen, red amazons, Ximchak barbarians, great magicians who create mythological beings like sphinxes, and the talking heads of Soorm. There is a flying construct in the shape of a bird named Bazonga, there is an enchanted forest which causes all who stay in it to fall asleep, and the Omega Triskelion, which is an entry to other worlds.
While these are not great novels, they are great RPG sourcebooks. In fact, each book has a map of the area and an appendix which gives more detail. In the first 2 books, the appendix is a glossary of names and terms. In the last 3, the appendix is a glossary of places. Also, do not overlook the many footnotes in the books, which are mostly humorous and give additional information.
There are a couple of reviews for Warrior of World's End here and here and Grognardia mentioned Giant of World's End, which was written before this series and is set chronologically after the series. I have not yet read Giant but am looking forward to it.