"The Mathematics of Magic", Ludovico Ariosto's Orlando Furioso (with a brief stop in Samuel Taylor Coleridge's Xanadu from "Kubla Khan") in "The Castle of Iron", the Kalevala in "The Wall of Serpents", and, at last, Irish mythology in "The Green Magician". With "The Green Magician" the original collaboration ended, Pratt's early death precluding any additional entries. A final planned story set in the world of Persian mythology was never written, nor was a projected response to L. Ron Hubbard's misuse of their hero in his novella The Case of the Friendly Corpse (1941). (De Camp would finally address the latter issue in "Sir Harold and the Gnome King".) Reviewing the 1950 edition of The Castle of Iron, Boucher and McComas described the series as "a high point in the application of sternest intellectual logic to screwball fantasy.". Damon Knight characterized the series as "relaced, ribald adventure ... priceless," saying that "no fantasy reader should be without them." In 1977 Richard A. Lupoff described the series as "whole planes above the hackneyed gut-spillers and skull-smashers that pass for heroic fantasy." Second series De Camp was reluctant to continue the series on his own, feeling that the collaboration with Pratt had a flavor impossible for either of them to duplicate alone. When he finally did revive the series in company with younger authors nearly forty years later, this impression was seemingly borne out; while not without interest, his own solo contributions to the second series exhibit a wryer, more cynical view of the worlds toured, and the protagonists' problematic use of magic is abandoned. His interest appears to have shifted to debunking the less credible aspects of the universes visited, rather than taking these as a given and extrapolating the fantasy worlds' physical or magical laws from them, as in the previous sequence. On the other hand, some of the new authors made efforts to duplicate de Camp and Pratt's original achievement, exploring fresh venues where their heroes once again have to learn the world's fundamental magical rules from the ground up. Holly Lisle ("Knight and the Enemy"), John Maddox Roberts ("Arms and the Enchanter") and Tom Wham ("Harold Sheakespeare") were among the authors who recreated the original formula. The initial impulse for the continuation may have been the successful adaptation of the characters into Tom Wham's authorized gamebook adventure Prospero's Isle, published by Tor Books in October 1987 (the basis of