C.S. Lewis is may favourite author. If you liked The Great Divorce you'll likely enjoy the rest of his fiction. The next closest in style and content are probably The Srewtape Letters and his space trilogy (Perelandra, Out of the Silent Planet, and That Hideous Strength). The seven book series of the Chronicles of Narnia is probably best purchased as a box set, and while often considered for a younger audience is actually pretty deep with layers of meaning. My favourite of his fiction was also his favourite, Till We Have Faces, a re-appropriation of the myth of Cupid and Psyche, and a bit more for an adult audience, with more obscure Christian overlays.
But yeah, one really shouldn't miss his non-fiction works, and for a bit of a mix it's best to go ahead and spring for something like The Complete C. S. Lewis Signature Classics (containing in one volume Mere Christianity, The Screwtape Letters, The Great Divorce, The Problem of Pain, Miracles, A Grief Observed, & The Abolition of Man).