How to add additional edition of boxed set?

[Buddhagan]

I have a 4-book set of the Time Quartet by Madeleine L'Engle (A Wrinkle in Time, A Wind in the Door, A Swiftly Tilting Planet, Many Waters). They were purchased together as one boxed set. Unfortunately I do not have the sleeve that they came in to check to see if the set itself had its own ISBN. Also it seems that this boxed set has disappeared off the Internet; I could find no pictures.

Publisher: Quality Paperback Book Club / Farrar, Straus, & Giroux Inc
Date: 1997
Cover Illustrator: Robert Goldstrom

Separate ISBNs for each book and page count:
A Wrinkle in Time 9780965846844, 203 pgs
A Wind in the Door 9780965846868, 211 pgs
A Swiftly Tilting Planet 9780965846851, 278 pgs
Many Waters 9780965846875, 310 pgs