Millennial Muslims of a certain stripe will recognize this unassuming book, written by one of the pioneers of Muslim fiction.
"Isabella: A Girl of Muslim Spain" by Yahiya Emerick was published in 1996, and tells the story of Isabella, the daughter of a Christian Cardinal in medieval Muslim Spain. Overhearing two Muslims discussing a point of Christian theology, Isabella is motivated to initiate a Cordoba-wide religious debate between the Church and the Muslim population... including the dashingly handsome Umar, whose overheard question spurred the entire chain of events.
Swept into an intrigue of Christian theology, Church panic, and a dastardly secret society, Isabella finds herself on a spiritual journey to the truth of Islam (and a thrilling rescue mission!).
I had adored this novel at age 12 and re-reading it 17 years later was both nostalgic and entertaining. The writing isn't as good as I'd remembered, but in truth, it had been revolutionary for its time. It's still got the bones of a good story, and with some tweaking, it has the potential for much better.
Also, interesting tidbit - turns out the original story was written by Maulana Muhammad Saeed Dehlvi, as a rather different take on the usual theological treatises for da'wah. (I know little else about it, though it would be interesting to learn more!)