Jack Chalker has written several series that I couldn't stop reading.
The Four Lords of the Diamond
| Lilith: A Snake in the Grass | Cerberus: A Wolf in the Fold | Charon: A Dragon at the Gate | Medusa: A Tiger by the Tail |
The Quintara Marathon
| The Demons at Rainbow Bridge | Run to Chaos Keep | Ninety Trillion Fausts |
The Saga of the Well World
| Midnight at the Well of Souls | Exiles at the Well of Souls | Quest for the Well of Souls | The Return of Nathan Brazil | Twilight at the Well of Souls | The Watchers at the Well |
The Watchers of the Well
| Echoes at the Well of Souls | Shadow of the Well of Souls | Gods of the Well of Souls | Ghost of the Well of Souls |
The Wonderland Gambit
| The Cybernetic Walrus | The March Hare Network | The Hot-Wired Dodo |
The Rings of the Master
| Lords of the Middle Dark | Pirates of the Thunder | Warriors of the Storm | Masks of the Martyr |