The Castle of Iron (The Incompleat Enchanter #2) by L. Sprague de Camp, Fletcher Pratt

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The Mathematics of Magic—it was the greatest discovery ever. Or so thought Professor Harold Shea. With the proper equations, he could instantly transport himself and his long-suffering colleagues back—or sideways—in time to all the wondrous lands of ancient myth and legend.
But slips in time were a hazard, and Shea's magic sometimes backfired. When it did, the results were usually bizarre at best—and potentially lethal at worst. The imaginary lands to which Shea travelled held countless real dangers that even the magic equations couldn't predict.