Lord Kalvan of Otherwhen

Blurb

Lord Kalvan of Otherwhen is a 1965 science fiction novel by H. Beam Piper and is part of his Kalvan series of stories, which is part of his larger Paratime series. It recounts the adventures of a Pennsylvania state trooper who is accidentally transported to a more backward parallel universe. It is Piper's last science fiction novel.
The book is an expanded version of the novelette Gunpowder God, which had been published in 1964 in Analog Magazine. Gunpowder God itself is a Paratime-series rewrite of the unpublished story "When in the Course", which takes place in the Terro-Human Future History milieu.