Tolkien did not initially want his trilogy to appear in so “degenerate a form” as the paperback book. What happened is that Donald Wollheim, then editor-in-chief of Ace Books, released an unauthorized edition of LOTR in 1965, believing, or claiming to believe, that the soon-to-be literary phenomenon was in the public domain. The Ace edition, being affordable at 75¢/book, sold extremely well, and Tolkien immediately embraced the paperback medium. Ballantine’s revised and authorized edition, 95¢/book, appeared in October, 1965 (The Fellowship of the Ring and The Two Towers) and November, 1966 (The Return of the King). Said Tolkien to his son in October of 1965:
Campaign in U.S.A. has gone well. ‘Ace Books’ are in quite a spot, and many institutions have banned all their products. They are selling their pirate edition quite well, but it is being discovered to be very badly and erroneously printed; and I am getting such an advt. from the rumpus that I expect my ‘authorized’ paper-back will in fact sell more copies than it would, if there had been no trouble or competition.
Wollheim’s unscrupulous maneuver—he was eventually forced to pay Tolkien the royalties he deserved—was the single most important event in the popularization of the fantasy genre.
You can see the spines and back covers of the original Ballantine editions at Tolkien Collector’s Guide, where I found the images above. The cover artist is Barbara Remington.