The ISBN of Marion Zimmer Bradley's Darkover is 0-88677-593-0.
The ISBN of Leroni of Darkover is 0-88677-494-2.
The ISBN of Towers of Darkover is 0-88677-553-1.
The ISBN of Domains of Darkover is 0-88677-407-1.
The ISBN of Snows of Darkover is 0-88677-601-5.
The ISBN of Renunciates of Darkover is 0-88677-469-1.
The ISBN of Red Sun of Darkover is 0-88677-230-3.
The ISBN of Four Moons of Darkover is 0-88677-305-9.
The ISBN of Free Amazons of Darkover is 0-87997-722-1.
The ISBN for The Mists of Avalon by Marion Zimmer Bradley is 978-0345350497.
The ISBN of "The Melting of Maggie Bean" by Tracie Vaughn Zimmer is 978-0689848787.
I believe you might be referring to the book by Christine Marion Fraser. The girls are named Kirsty and Jean. The publisher is Lions. The ISBN-10 number is 0006746233, the ISBN-13 number is 978-0006746232.
The ISBN of The Summoned is 0-7434-0700-8.