Envisioned by the husband-and-wife team of Ian and Betty Ballantine, and edited by Lin Carter, it featured cover art by illustrators such as Gervasio Gallardo, Robert LoGrippo, David McCall Johnston, and Bob Pepper. The agreement signed between the Ballantines and Carter on November 22, 1968, launched the project. In addition to the reprints comprising the bulk of the series, some new fantasy works were published as well as a number of original collections and anthologies put together by Carter, and Imaginary Worlds, his general history of the modern fantasy genre.
The series was never considered a money-maker for Ballantine, although the re-issue of several of its titles both before and after the series' demise shows that a number of individual works were considered successful. The Ballantines supported the series as long as they remained the publishers of Ballantine Books, but with their sale of the company to Random House in 1973 support from the top was no longer forthcoming, and in 1974, with the end of the Ballantines' involvement in the company they had founded, the series was terminated.
After the termination of the Adult Fantasy series, Ballantine continued to publish fantasy but concentrated primarily on new titles, with the older works it continued to issue being those with proven track records. In 1977, both its fantasy and science fiction lines were relaunched under the Del Rey Books imprint, under the editorship of Lester and Judy-Lynn del Rey. Carter continued his promotion of the fantasy genre in a new line of annual anthologies from DAW Books, The Year's Best Fantasy Stories, also beginning in 1975. Meanwhile, the series' lapsed mission of restoring classic works of fantasy to print had been taken up on a more limited basis by the Newcastle Forgotten Fantasy Library, launched in 1973.
All books in the "series proper" bore a distinctive Unicorn's Head colophon on the cover and included an introduction by Carter.
Precursors, August 1965 to April 1969
Ballantine published these fantasies and fantasy criticism before hiring Carter as consultant. Some were labeled "A Ballantine Adult Fantasy" on the first Ballantine cover. Later reprints of some bore the Unicorn's Head colophon.
The Mezentian Gate, E. R. Eddison (April 1969, with "A Ballantine Adult Fantasy" on the cover)
The series proper, May 1969 to April 1974
Volumes published as part of the series, based on a listing by Lin Carter in Imaginary Worlds: the Art of Fantasy with the addition of books new to Ballantine published under the Unicorn's Head colophon thereafter. In Carter's list, the books were numbered in the order of their publication; subsequent numbers supplied on the same basis. These numbers do not appear on the books themselves.
Two volumes published after retirement of the Unicorn's Head colophon were evidently intended for the series. The first has a Carter introduction and the second completes a set of four begun under his editorship.
Other works considered by Carter for inclusion in the series
Carter intended to reissue or compile these books for the series, according to statements in his introductions to other books in the series and lists discovered among his effects after his death or elsewhere. A few were later issued in the Newcastle Forgotten Fantasy Library, a fantasy revival series similar to the Ballantine Adult Fantasy series published between 1973 and 1980.
City of Sorcerers, Henry Kuttner (combined with Campbell's The Elder Gods, included in a list of "recent" series titles in The Man Who Was Thursday but not in fact issued). City of Sorcerers is an alternate title for the Kuttner novella "Lands of the Earthquake"