I first met H. Beam Piper in the pages of Analog, the science-fiction magazine, in the mid-'60's. The 3 installments of the Pennsylvania State Trooper series are in this volume and their merit as storytelling is obvious.
This volume re-unites those three stories about Corporal Calvin Morrison, later Lord Kalvan, who but for the sideways time travel aspect is pretty much in a late medieval story, albeit in the Susquehanna River country of Pennsylvania -- at, least, in this new time-line.
Other (non-Kalvan) stories in this volume are an undiscovered joy, notably the first, "He Walked Around the Horses," which is one of the best SF short stories of this genre I've seen -- indeed, worthy of an Ursula LeGuin in its plotting and concept.
And it is an original concept: _lateral_ time travel, in which multiple histories exist on parallel timelines. So it can be that someone from modern Pennsylvania -- Cpl. Morrison of the Pennsylvania State Police -- winds up in a timeline in which the Aryan peoples migrated east across Asia and the Pacific rather than west into Europe, and now are a civilization roughly equal to Europe ca. the Thirty Years' War. The Pennsylvania scenery may be the same for Calvin Morrison, but the kingdoms may not be -- especially as the dominant religion has monopolized the formula for gunpowder. Monitoring all these timelines is a more-advanced civilization on a more-advanced timeline, with Paratime Police to monitor any cross-time incidents, and who take a skeptical interest in Kalvan's new adventures.
This author was a master, really, and we shall not see his like again anytime soon.