A Knight in Shining Armor, by Jude Devereaux, Pocket Books, 1989.
This book was my guilty pleasure in my early twenties. I probably read it at least three times back then. After that, I put it away and didn't read it again for several years -- until now.
Most of what enchanted me in the first place still at the very least held my interest. I remember being struck then how well this book was researched for it 'just being a romance'. I still found it well-researched. I may have found the motives of the main character a little difficult to understand now, but in my twenties, they would have made perfect sense to me.