This the remarkable story of John Morse, told in his own words, an Englishman caught in Germany at the outset of WW1.
He was on business in what was then the Eastern border of Germany (modern day Poland) when Germany was about to declare war on the Entente Powers. His story starts with him slipping through German lines during their initial crossing of the Russian border. He eventually finds the Russian army and stays on to fight the Germans. In the 19th Century he would have been described as a gentleman volunteer, although I doubt the Russian army would recognise the concept. He fought with a Cossack unit, an artillery battery and several infantry units for almost a year before being captured, then escaping, before making his way back to England.
His observations of the war in the East, while very English of the period, are fascinating. He gives great descriptions of small and large scale actions, the impact of fortifications and artillery as well as the much more fluid nature of the war in this sector. While not a professional writer, the book is still a good read and an essential source for anyone interested in the period.