There's another one that I haven't read called the, The Water of the Wondrous Isles and a follow up to The Wood Beyond the World called The Well at the World's End.

Translations of icelandic sagas, and of course, Tolkien went the same route with Lord of the Rings - history of ideas. Wonder if Morris knew the Ygradsil Tree myth, Tolkien must of because of the Ents....