If memory serves, the first book that I purchased for myself was an anthology titled Great Stories of Space Travel, edited by Groff Conklin.
It is a decent collection of science fiction stories by high-profile authors taken from the 1940s and early 1950s. The anthology was published in 1963. The copyright page on my copy indicates that it is "A Tempo Books Original", but doesn't seem to have any information regarding which printing it comes from. I suspect that I purchased it in 1977 or 1978.