L. Sprague de Camp’s 1963 The Search for Zei / The Hand of Zei is the Ace Double edition of the second book in de Camp’s Krishna planetary adventure series.
Krishna! Exotic planet of adventure! A place that has little day-to-day relevance to ghostwriter Dirk Barnevelt, living as he does a life of quiet oppression under the iron rule of his mother on Earth, twelve light-years away. The closest Barnevelt gets to adventure is writing up the exploits of interplanetary explorer Igor Shtain.
Then Shtain vanishes.
(On the one hand, spoilers; on the other, this book is ancient.)