The Conspiracies of Rome - Richard Blake
Apparently this is the first book of a trilogy, set in Rome at the time of the fall of its empire.

Our protagonist is Aelric, who is looking back at his life from the perspective of being an elderly monk in Jarrow, telling tales of when he used to be a lusty (literally) young man. One of a family whose inheritance has been stolen, Aelric finds himself in trouble for having his way with the wrong woman and is forced to flee the country, reluctantly travelling to Rome with the missionary Maximinus.

The empire is in its death throes, overrun by invaders and struggling to maintain a grip on power while, in contrast, the power of the church continues to grow. In Rome, Aelric makes both friends and enemies after his fellow traveller is murdered and he determines to know the truth about who killed him and why.

I'm a bit of a fan of historical detective stories, but sadly I guessed who was ultimately responsible for what had happened before the author revealed it, which is always a disappointment. At times, also, the anachronisms got a little too much as the story got bogged down in very unRoman attitudes and mores, as well as the feeling the author is trying to shock and not always managing it. The trilogy continues in [b:The Terror of Constantinople|6378908|The Terror of Constantinople|Richard Blake||6566706], but I'm not sure I'll bother.