The illustrator is Mary Fairburn, whom Tolkien himself observed had a rather hobbitish name, and the story of how she came to illustrate Tolkien is an interesting one -- perhaps rather more interesting than the art itself.
As a young artist, Fairburn sent her work directly to Tolkien back in 1968 in hopes that she might be able to do an illustrated edition of THE LORD OF THE RINGS. Tolkien was much impressed by her work and even purchased some of it for himself when plans for the proposed illustrated edition fell through (not to be realized until many years later with the Folio Society and, later, Alan Lee editions). Fairburn herself didn't keep any of the art, but a number of pieces survive because she gave them to a friend who kept them all these years. This calendar collects together the surviving original pieces, most of which Tolkien saw and approved, plus her re-creation of some of the missing ones.
The best piece by far is the one used as the calendar's cover and again as the image for the month of April: the bottom half is Tolkienesque in its use of contour lines, while the top half is distinctly Sime-ish,* esp about the clouds and sky.
In general I much prefer the older pieces, which are more distinctive in style, as well as more surreal and slightly creepy: the Inn at Bree, the Old Forest, the Bridge of Khazad-dum, the Nazgul over Dead Marshes,** and an unusual Treebeard. Tolkien's favorite was the Mirror of Galadriel, perhaps due to its simplicity and her attention to background.
As an added bonus, the calendar also comes with an interesting essay by Paul Tankard giving a brief bio of Fairburn and a history of the project, quoting from her correspondence with Tolkien, and describing how it faltered under the complications and difficulties of both Tolkien's and Fairburn's lives. A more detailed version of this essay appeared in the TIMES LITERARY SUPPLEMENT at the time of the artwork's re-discovery and can be found at the following link:
All in all, it's good to have Fairburn's story, and the previously unpublished passages from Tolkien's letters. It's nice that Fairburn, now 82 and living in Australia, gets recognition and a little glory for her work. And it's good to see the art, which stands up v. well by comparison with what Tolkien art there was in the late Sixties -- this is even earlier, I think, than Tim Kirk, and of course both earlier and far better than The Hildebrandts.
*I'm thinking about the late Sime here, after he'd retired from illustrating and worked away in his studio at Worplesdon on whatever image pleased him, most strikingly a portrait of John on Patmos.
**this picture is accompanied by a rather defensive note to the effect that "Gollum is painted black, in accordance with at least seven references in The Lord of the Rings" --presumably whoever wrote this note had become aware of Tolkien insistence, in his comments on Pauline Baynes' disastrous attempt to portray Gollum, that Gollum is pale-skinned.