"Giovanni's room" may well be the greatest gay novel ever written--it's a masterpiece, and absolutely perfectly constructed. I also really like his novel before that one, "Go Tell it On the Mountain."
I tried "Another Country" and thought it was too sprawling and unfocused--it seemed almost as if it were by another novelist who wasn't as good. I've not read the other novels, which are not as famous as the first three nor are they discussed as much (although there was a film of "If Beale Street Could Talk" a few years ago that I missed). Are they any good?