The Warlock in Spite of Himself
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Book Two in the Warlock of Gramarye series by Christopher Stasheff, published in 1969.
From the Publisher:
"The novel that launched the epic Warlock series."
"In an interstellar romp that proves science and sorcery can mix, only hard-headed realist Rod Gallowglass can save the people of Gramarye from their doom by becoming — The Warlock in Spite of Himself — if only he believed in magic."