A Murder on the Appian Way is the fifth in Steven Saylor's Gordianus series. It doesn't go on sale very often, so even though £2.99 isn't a great price for decades-old backlist, here you go...
Kindle UK:
https://www.amazon.co.uk/dp/B007JKEVAE
Kobo UK:
https://www.kobo.com/gb/en/ebook/a-m...e-appian-way-2