This is a really old one. First published in Astounding Science fiction in 1947 one would expect it to be full of anachronisms, dated mores and all sorts of silliness. It does have old sfnal idea of Venus being a planet occupied by a massive hostile jungle, but this can be found in SF books published 20 or more years later and I wonder if here might be the first time it was used. The human race, having destroyed Earth in a nuclear war, is now resident in ‘keeps’ under the Venusian seas. It is sinking into decay under the overly cautious rule of immortals and needs someone to pull it out of that. Enter Sam Reed . . . I enjoyed this very much and didn’t find much in the way of the stuff that has made me wince in other SF books. As for Venus being a jungle planet it was easy enough to forget the reality.
Recommended, even though this is 71 years old.