Universal (later Star), a UK imprint, published the early Witch World titles in the late 1970s. All featured art by Rodney Matthews. In an email from someone at Rodney Matthews Studios, I was told that he illustrated the following titles: Witch World, Web of the Witch World, Three Against the Witch World, Warlock of the Witch World, Sorceress of the Witch World, Year of the Unicorn, and Trey of Swords. The email did not mention Spell of the Witch World, which has also been identified as Matthews' work.
There is some question as to whether or not this edition of Sorceress of the Witch World was ever actually published. Discussion on the Andre Norton Forum at SF-Fandom between some very knowledgeable collectors seems to suggest that it was never published. It's difficult to trace, as Tandem, who published earlier copies, became Universal, and the same identifying numbers were used on both versions.
All of these books featured art that wrapped onto both covers. For full cover art, see the Rodney Matthews section under Cover Artists.