Agricola by Cornelius Tacitus – Free Ebook
Agricola was the Roman general who first conquered Scotland around 75AD. His father-in-law Tacitus wrote a book detailing his battles which is how we know today how that was done, although some of the details may have been talked up.
The book is available for free at https://www.gutenberg.org/ebooks/7524 with a kindle and other versions.
See Agricola (Tactitus) on Wikipedia for the story of the book.