If I understand correctly,
Drigel is referring to artwork done by JRR Tolkien himself and used for some of the edition covers.
Sketches and complete drawings of some of the works
Drigel refers to can be found
here. One of my favourite drawings by Tolkien has always been 'Bilbo comes to the huts of the Raft-elves." It shows so much similarity to Art Nouveau style--I find it fascinating that Tolkien tinkered in such a 'modern' style.
There are several others sites on the web which host Tolkien's own artwork.