There's a 5 year gap between the end of the last novel in this 6 book series (Caesar's Women) and this one: Caesar is in Gaul, and finds his ties in Rome being cut. Instead he throws himself into the Gallic campaigns which are described in minute and enthralling detail (based on Caesar's own commentaries).
I don't know how McCullough does it, but she manages to render military campaigns, legion's rebellions etc absolutely fascinating! This isn't by any means an objective look at either Caesar or Roman imperialism, and she is unashamedly on Caesar's side, but somehow it works fabulously.
Back in Rome the Senate led by the vacillating Cicero and neurotic Cato and undermining Caesar, and the book leads inevitably and inexorably to Caear's crossing of the Rubicon.
I can't praise this book enought - not as history (which it doesn't purport to be, though it does stick to the sources - albeit in an interpretative way) but as sheer story-telling.