This is the front of a tri-fold business reply mail card that was tucked away inside a 1968 Pocket Books paperback of Zane Grey's "The U.P. Trail." The volumes advertised are part of the 1960s Zane Grey set published by Walter J. Black. There were a total of 73 titles in the set, according to Zane Grey's West Society, which states:
"The Walter J. Black Editions ... were bound in red and tan cloth and originally came in a rice paper cover. Most of these books have little value, but the last thirteen ... are substantially rarer and command higher values. Two titles, The Wolf Tracker and The Camp Robber are worth a fairly high value. The Wolf Tracker is almost impossible to find in the original Harper's Edition, so this title is, in many cases, collected by collectors who feel they will never find the original first. The Camp Robber is a First Edition in the Walter J. Black series. It was never published anywhere else other than a magazine serial. It is therefore highly collectible. ... [M]ost of these titles in average condition can be purchased for $5 - $6 per book."
Of course, when they were originally published in the 1960s, you could get the first three in the set for JUST $1.00. Additional volumes were $2.89 apiece, "plus a few cents mailing charges," according to the advertisement.
Here are some additional images from the tri-fold card.
The advertising copy describes the books in more exciting terms than the Zane Grey's West Society website. It states: "These De Luxe Volumes Are Bound in Glowing Shades of Red, Tan, Blue ... and Stamped in Genuine Gold!"
In fact, all of the advertising copy is fairly "glowing":
"Is a rip-roaring story of the Old West your 'meat'? Then you'll want these three great Western thrillers by ZANE GREY - because every Zane Grey Western is jam-packed with blazing gunplay, breath-taking action, blood-tingling excitement! ... How proud you will be to display these luxurious volumes! Just picture them in your home - and imagine the world of pleasure they will open up for you and your family!"
And if that's not enough to convince you, I will leave you with The Floating Head Of Zane Grey...