The Chronicles of Chrestomanci by Diana Wynne Jones?
They definitely still exist! They just have different covers now.
https://www.amazon.com/Chronicles-Chrestomanci-Vol-Conrads-Pinhoe/dp/0061148326/ref=pd_bxgy_14_3/139-1500460-9717724?_encoding=UTF8&pd_rd_i=0061148326&pd_rd_r=07525ead-0f06-4b06-b2d0-f52710843bc5&pd_rd_w=uYimR&pd_rd_wg=7fOoI&pf... (Note: This volume was not out when I was a kid, so your husband may or may not have read it.)
If you want the cat covers for the nostalgia factor (those were the ones I got as a kid, anyways!), looks like you can find them on eBay-- https://www.ebay.com/itm/DIANA-WYNNE-JONES-vol-1-2-CHRONICLES-OF-CHRESTMANCI-lot-of-2-paper-back-books/233375992934?hash=item36564b0c66:g:xCIAAOSw9MRdrb7-
(I don't see the cat covers on Thriftbooks right now, but I bet they would show up if you kept an eye out.)
I'm not sure which book in the series, but that sounds like The Chronicles of Chrestomanci series by Diana Wynne Jones?