Ace Double H-22 (1967): Lord of the Green Planet, bound with Five Against Arlane.

Lord of the Green Planet, by Emil Petaja. Cover by Kelly Freas, frontispiece by Jack Gaughan; 118 pages. The first of 2 books set on the "Green Planet."
Astronaut Diarmid O'Dowd, having crashed his ship through the green barrier, finds himself cast into the role of a doomed hero in the Deel's song of ancient Ireland- at the will of the self-established god of this world. O'Dowd doesn't like the destiny layed out for him, so he sets out to break the pattern, with the help of Old Grane, the Wees, the silkies, and Fianna-of-the-Dreaming-Lips.

(Bound with) Five Against Arlane by Tom Purdom. Cover and frontispiece by Jack Gaughan; 136 pages.
David Jammet ruled Arlane with fear, drugs, and controllees- people whose wills were subverted to Jammet's wishes. Four out-worlders: Migel, Rolanda, Zach, and Sylvia befriend Anata, the only Arlanian willing to give up her chance at eternity, work to break the rule of Jammet.

This is one of Purdom's early novels from the Sixties - he is still active in the field, contributing a couple dozen stories to Asimov's from 1990 to 2012.