This is such a well-known primary source that I kept having déjà lu all through it. And, indeed, I had already read these instances in secondary works.
There is a bit of haziness about the text itself, which is not uncommon in memoirs from the time. Benjamin Harris was a real rifleman, and the published text is based on his recollections, but he did not write the book himself. Henry Curling, who was an officer of the same regiment but more recently than the events described, became a prolific popular writer. He met Harris and got him to tell his story, then Curling wrote it up and saw it published. We have no way of knowing how much of this is Harris and how much is Curling; and certainly the choice of material is mostly Curling.
But, in any case, there is a strong feeling of authenticity to this document. Harris tells things about himself that many would hide, and he makes observations about how battle feels (especially the dissociation from death) that are rather "ahead of their time" in war memoirs.
Harris was conscripted to fight Napoleon, in a regular unit, the 66th Regiment of Foot. While in Ireland with the 66th he saw a group from the 95th Rifles (an elite unit) and loved their uniforms and their attitude, so he got permission to join them. He was also a cobbler, and so he had to carry his shoemaker's gear with him at all times, and was often kept up the night before a battle repairing shoes (even when he himself was barefoot) and boots. He did get some extra money for this, as well as from picking over dead bodies on the battlefield, which served him in the end.
He campaigned in Denmark and then in the Peninsular War, serving under He-Who-Would-Be-Wellington, as well as Moore and Craufurd. He was part of the reinforcement that arrived at the front after a long march, just before Moore began the disastrous Retreat to Corunna, though his group diverged, retreating to Vigo under Craufurd. Napoleon had arrived personally in Spain after Moore had surprised and defeated Soult, and Napoleon had a very large, fresh army with him. Moore fled, moving too fast to bring up stragglers, abandoning equipment and damaged wagons, as they crossed the Galician mountains in the dead of winter. It all seems like a prelude to the Retreat from Moscow. Moore's group basically dissolved into a rabble along the way -- though it did partially reform for a final battle, when they got to Corunna and found no transports.
Craufurd's army suffered horribly, too, but Craufurd kept his folks together as a functioning unit. Yes, they had to leave the sick, exhausted and wounded behind, along with many wives and children. Yes, they took considerable losses from weather and from repeated rear-guard action. But they did much better than Moore, and one of the essential elements of this book is that Harris tries to explain how he did it. It was a mixture of strict discipline with clear signals that he was doing this all for them. Harris knows he's not quite explaining how it worked, but he knows it was a really big deal, and that Craufurd was a gifted leader.
The story of the Retreat alone would make this invaluable, but Harris served in another campaign for which he is an equally invaluable historical source, including for debates that rage to this day. This was the ill-fated Walcheren Campaign, in which the British tried to distract Napoleon from the Austrians, by seizing part of the Netherlands. Alas for the British, Napoleon had already defeated the Austrians at Wagram, before they even landed. But worse was still to come. The campaign began as a cake-walk, the Brits landing with little resistance and occupying Flushing and a couple of islands. And then everybody, and I mean pretty much everybody, got very, very sick. The Army that arrived was 42,000 strong, and 8,000 of them would die in short order, and something like twice to thrice that were sickened. Some think that the majority of those "sickened" would ultimately die, off the books, of the disease. Our best guess today is that it was a combination of malaria and typhus and typhoid and dysentery. Swollen spleens were common, which says malaria, but that wouldn't account for the numbers.
Harris's account of the event and the aftermath (he spent years in various hospitals and camps) is shocking and powerful, despite not being very long.
I'm sorry I took so long to get to the original. Highly recommended.