We’re talking about books disappearing: books originally printed only in paperback—and books that are no longer in issue. I’m going to be listing, among others, people I knew personally, and series I’d expect an sf fan to have read, or looked at, or have some familiarity with.
Let’s start with:
Clifford “Cliff” Simak: City.
Gordon R “Gordie” Dickson. The Dorsai stories.
Hal Clement. Mission of Gravity
Lin Carter. Sword and sorcery…from the Old School.
Poul Anderson: many, many, from SF to near-historical.
Robert Howard. Conan, Kane, Kull.
C.L. Moore: many stories.
Leigh Brackett: many stories.
Edgar Rice Burroughs: Tarzan, John Carter of Mars
Joe Green: Tom Corbett, Space Cadet
Anthony Gilmore: Space Hawk.
Andre Norton: the Witchworld, et al; and Star Man’s Son.
Jack Vance: the Dirdir, the Pneume, etc.
Jerry Pournelle and Larry Niven as a team, the hell books.
Larry Niven: Ringworld
Joe Haldeman: the Forever War
Abbey and Asprin: Thieves’ World (give CC time)
Marion Zimmer Bradley: Darkover novels.
Anne McCaffrey: the Pern novels.
Roger Zelazny: Dilvish the Damned [story], Lord of Light, the Amber stories, Damnation Alley.

The point being—most of the younger people coming in may not even have heard of these.
Name more of the ones that are harder to find in print now.