Martin Padway is dropped suddenly into 6th-century Rome, and does his best to prevent the Dark Ages from happening. Written in 1939, this story basically spawned the alternate-history genre; it features meticulous attention to historical accuracy, along with a complete inability to take itself seriously. If you like the 163x series you'll like this.
My copy's the Baen reprint from the mid-90s, which includes David Drake's short story "To Bring the Light" (which was enjoyable enough, although it's fairly obvious where it's going from the start), and features one of their standard incredibly tacky cover designs.
ISBN 0671877364.