This seems to be Midnight at the Well of Souls by Jack L Chalker, its the first in the Well of Souls series, 10 books about the Well World
Well of Souls
- Midnight at the Well of Souls (1977)
Below average in height and unprepossessing in appearance, Nathan Brazil is an unassuming if cynical starship owner, carrying passengers and cargo for a living - hardly the sort of person to hold the fate of the entire universe in his hands.
But when Nathan detours from his route to answer a distress call, a hidden stargate hurls him and his passengers to the Well World, the master control planet for the cosmos. Billions of years ago, a godlike race unlocked all the secrets of space and time and remade the universe according to their grand design. Then they vanished and left behind the Well World to maintain the pattern of the universe.
Now someone is searching for the planet's hidden control room - the Well of Souls - to seize control of the cosmos. Nathan must stop them, but this planet causes bizarre metamorphoses in visitors, changing them into centaurs, mermaids, and giant insects seemingly at random. As a result, Nathan no longer recognizes his friends, let alone his unknown enemies. His task seems hopeless - until his long-suppressed memories begin to return and he realizes who he really is.
- Exiles at the Well of Souls (1978)
- Quest for the Well of Souls (1978)
- The Return of Nathan Brazil (1979)
- Twilight at the Well of Souls: The Legacy of Nathan Brazil (1980)
- The Sea Is Full of Stars (1999)
- Ghost of the Well of Souls (2000)
Watchers at the Well
Echoes of the Well of Souls (1993)
Shadow of the Well of Souls (1994)
Gods of the Well of Souls (1994)