One book? One book?? Inconceivable!
I'm going to nominate a series, Colleen McCulloch's Roman series. I think it starts with the First Man in Rome.
It starts in the later days of the Roman Republic just prior to the rise of Gaius Marius, and ends at the start of Octavian / Augustus Caesar's reign as emperor. They're all big meaty books with massive glossaries, intensively researched, and written with quite a wonderful sense of character and humour.
She does seem to have a healthy dose of worship for Julius Caesar though.
They're wonderful books, worth the read, and definitely up there in my Favourites list.
If anyone else likes big meaty historical novels, please also try Edward Rutherford. Sarum, New Forest, Dublin, New York, etc. Brilliant reads.