One of my favorite SF writers when I was growing up was Clifford Simak. I must have read just about every one of his Novels that I could find. I primarily recommend "City" and "Waystation". Recently, I started going back and reading a bunch of Heinlein novels that I never read before, none of which I want to really recommend (none of them were bad, per se, but none were worth mentioning). Instead, I will recommend some classic Heinlein: "Stranger in a Strange Land", "The Puppet Masters", "The Moon is a Harsh Mistress", "The Past Through Tomorrow", and "Time Enough for Love". The last two should be read in that order -- "The Past through Tomorrow" is a short story collection that kind of fits an alternate history, and "Time Enough for Love" follows it sequentially. There's a lot more that's good, but you can start with those.