Dies the Fire by S.M. Stirling
Lucifer’s Hammer by Larry Niven and Jerry Pournelle
Alas Babylon by Pat frank

Lucifer’s Hammer was written in 1977 and Alas Babylon was written in 1956 (I think) both are well written and some of my favorite books but they were written by men who grew up in a different era and there writing reflect attitudes and ethics that are sorely lacking in today’s society and as a result they are IMHO overly optimistic about what would happen. Dies the Fire is the first book on a series but well worth the time.