I've only recently read 'Downward to the Earth' and it was a very good story, showing a very strange world (even if the cover of the book had elephants on it!), a guilty past and how the interactions the humans had had with the aliens and how significant they were. Well worth reading.Both are very good; Man in the Maze was actually the first of his I read, but personally I'd go for Nightwings, which is one of my all-time favourite Silverbergs.
If you're looking for other recommendations, Downward to the Earth is another that I thought was fabulous. Even though I guessed the 'big reveal' early on, it still didn't stop me enjoying what was an excellent evocation of an alien world and the humans interacting with that environment. Dying Inside is another must-read -- totally different and very intense, but quite superb. Then there's Thorns... (sigh)