The Allegory of Love is one of the seminal works of medieval criticism. It’s still very relevant, and I remember using it in my master’s thesis years ago—I was writing about Chauceriqn allegory and poetics at the time. Now I deal more with early medieval English where Lewis’s work isn’t as important—but it is still a great work price of scholarship and he was so young when he wrote it!
The Allegory of Love is indeed a great book. I never read anything by Lewis that I didn’t enjoy or profit from.
I like all his fiction, including Narnia and the SF trilogy, but I think that Till We Have Faces is his greatest novel.