I will nominate Quo Vadis
by Henryk Sienkiewicz. Sienkiewicz won the Nobel Prize for literature in 1905 for this and other epic novels. This book is in the public domain with with ebooks available in German and English here at MR. From HarryT's description of the book:
Another great Hollywood "blockbuster", but how many have read the book?
"Quo Vadis: A Narrative of the Time of Nero", commonly known as "Quo Vadis", is a historical novel written by Henryk Sienkiewicz. Quo vadis is Latin for "Where are you going?" and alludes to a New Testament verse (John 13:36). The verse, in the King James Version, reads as follows,
"Simon Peter said unto him, Lord, whither goest thou? Jesus answered him, Whither I go, thou can not follow me now; but thou shalt follow me afterwards."
Quo Vadis tells of a love that develops between a young Christian woman, Ligia (or Lygia), and Marcus Vinicius, a Roman patrician. It takes place in the city of Rome under the rule of emperor Nero around AD 64.
Sienkiewicz studied the Roman Empire extensively prior to writing the novel, with the aim of getting historical details correct. As such, several historical figures appear in the book. As a whole, the novel carries a powerful pro-Christian message.
Published in installments in three Polish dailies in 1895, it came out in book form in 1896 and has since been translated into more than 50 languages. This novel contributed to Sienkiewicz's Nobel Prize for literature in 1905.
Several movies have been based on Quo Vadis. The most famous movie is the Hollywood production "Quo Vadis" filmed in 1951.
Yes, a Christian theme (not inappropriate for December), but having read it years ago I can say it is also an entertaining novel about the court of Nero at that time in history. Even atheists can enjoy.