In a public school in Mississippi around 1977 I came across a series of science fiction juvenile books.  When I say "series", I mean that they were by the same publisher and had the same binding...but they did not share characters or setting or plot.  The books may have been fairly short (30 pages? 100?)

One book was about people living in an underground city who ventured to the surface and found that the apocalypse that had caused their ancestors to tunnel down was not nuclear war, but vast mountains of garbage (no, this is not "This Time of Darkness" by H M Hoover!)

Another was about astronauts who land on a planet.  While wearing space suits they encounter a woman who walks around suitless.  She convinces one of them to take off his helmet because "the air is fine".  He does, and dies.  It turns out that she is actually some sort of alien energy vampire who feeds on his death.