If you had only three Catholic apologetical books that you wanted Protestants to read, what would they be?
Why, the ones I wrote, of course! 🙂
Actually, it’s hard to say since it depends on what the issue is for the particular Protestant. But I would start with By What Authority? since sacred tradition is at the heart of the divide between Catholics and Protestants. Then, it would be up to the individual. I wrote This is My Body and Mary, Mother of the Son because the Eucharist and Mary are two huge hurdles. For others, it might be the papacy, or scandals in the Church, or other stuff.
Beyond that, I’d survey some of the big Catholic apologetics sites for suggestions. Also, I’d strongly recommend looking beyond apologetics to just good spiritual writers without an argumentative axe to grind. There’s no such thing as Protestantism, just Protestantisms as varied as the number of Protestants. So having a wide array of materials is essential in order to tailor your appeal that particular person with those particular issues and questions.