Competition for the Balllantine Adult Fantasy Series?

The Ballantine Adult Fantasy series proper began with the May 1969 publication of The Blue Star by Fletcher Pratt. Over the next year Ballantine published one or two additional titles per month, and other publishers were certain to have noticed the success of the series.

Paperback Library was one such outfit. It had been founded in 1961 and by the end of the decade it was known for publishing a lot of science fiction, a lot of westerns, and a lot of tie-in books to the popular television series Dark Shadows. Their attempt to move into the fantasy market was in the end limited to four titles, all published in 1970. It was apparently not very successful. Warner Communications bought the firm in 1972, and by 1972 the Paperback Library output had descended to a trickle, with the final books coming out in early 1973.

The fantasy imprint apparently had no name, beyond the label of "Paperback Library Fantasy Novel" which appeared at the top of the front cover of each book. However, like the unicorn masthead of the Ballantine Adult Fantasy series, these four books each have a device of a centaur wielding a bow and arrow.

The four titles are as follows:

The Citadel of Fear, by Francis Stevens (August 1970).
The Serpent by Jane Gaskell (October 1970)
Atlan by Jane Gaskell (October 1970)
The City by Jane Gaskell (October 1970)

The Francis Stevens book has an introduction by Sam Moskowitz, riddled with his usual errors (this introduction is the source of the erroneous idea that H.P. Lovecraft had written an appreciation of Francis Steven back in 1919. Lovecraft's fellow Providence resident Augustus T. Swift had in fact written a letter to Argosy in 1919 in praise of Stevens. Moskowitz simply presumed that Augustus T. Swift was probably a pseudonym of Lovecraft's, and Moskowitz began his introduction by quoting a long passage from Swift's letter, attributing it to Lovecraft, and not even mentioning Swift's name).  The cover artist's name is not printed anywhere, but you can clearly see the artist's signature, Steele Savage, in the art itself in the lower middle of the front cover.

The three volumes in Jane Gaskell's trilogy had previously been published as "science fiction" by Paperback Library in 1968. With the new October 1970 printings, they are each now labelled as "A Paperback Library Fantasy Novel." And they have new uncredited cover art, but I'm pretty sure the art is by Michael Leonard.

There is one further release from Paperback Library of potential interest. This is the September 1971 issue of The Tritonian Ring by L. Sprague de Camp. It was published not as a fantasy, but as science fiction. Yet the cover art (uncredited, but probably by David McCall Johnston, who did some covers in the Ballantine Adult Fantasy series, including those for the Evangeline Walton books), and the blurb comparing it to Tolkien's The Lord of the Rings, certainly give more of a fantasy vibe than a science fiction one. 

P.S.  I'm adding here the cover of the William Ready book on Tolkien as mentioned by David Bratman in the comments below. The cover art is clearly meant to recall Barbara Remington's mural published on the 1965 Ballantine editions of The Hobbit and The Lord of the Rings.