Victory of Janus by Andre Norton

Paperback. Condition: Good. 1966. Ace. 1st Printing. 

Waking suddenly and prematurely from their winter sleep on the planet Janus, Ayyar and his Iftin comrades (who had been Earth colonists before their transformation into Ifts) find a hostile force threatening their citadel, the great tree fortress of Iftsiga. It is soon apparent that their ancient enemy, an extraordinarily strong --- but never seen --- evil force, has risen again and is marshalling all its power to rid the planet of the peace-loving Ifts.