Help must find this book

[gegsie]

Its a paper back. About a policeman type person who checks on worlds to make sure they are progressing naturally. He has a ship that buries itself and he can put the ships computer into a real looking robot horse, but the computer has a glitch which gives the horse epileptic fits. He lands on a medieval world where he meets a woman called Gwendilon. Unfortunatly thats all I can remember. Hope someone can help

 

[Teresa Edgerton]

The Warlock in Spite of Himself by Christopher Stasheff