Setting up a C.S. Lewis Collection reading plan

Wow, great question! And I'm sure you'll get different answers. I don't think there is one strict order to read them in. But I would prioritize the following:

• Mere Christianity
• The Chronicles of Narnia (unfortunately not in this collection, but must reading; available on Kindle); and I would recommend reading them in their original published order, not in the chronological order that they're now published in. There are huge debates on that, but I prefer the unfolding of discovery about Narnia in the way it was presented to me as a child: 1. The Lion, the Witch, and the Wardrobe; 2. Prince Caspian; 3. The Voyage of the Dawn Treader; 4. The Silver Chair; 5. The Horse and His Boy; 6. The Magician's Nephew; 7. The Last Battle.
• Surprised by Joy (also not in this collection, surprisingly) - his autobiography
• Screwtape Letters

Second Tier (mostly his main theological works):

• The Four Loves (surprised that it's not in this collection)
• The Great Divorce (it's not about the dissolution of marriage!)
• The Abolition of Man
• Miracles
• The Problem of Pain
• The Weight of Glory
• A Grief Observed
• Reflections on the Psalms (not in this collection)
• The Space Trilogy in order (Out of the Silent Planet, Perelandra, That Hideous Strength)
• Letters to Malcolm: Chiefly on Prayer (also not in this collection)

Third Tier:

• God in the Dock (several of his other books of essays might also belong in here, but I'm actually not sure which ones I'd prioritize; there have been so many published collections, many of them with overlaps, and I don't remember which ones I've read apart from this one; I'd prioritize certain individual essays rather than a particular collection of them, actually; but I haven't read enough of them to give you a listing of my top priority essays)
• The Collected Letters of C.S. Lewis (I might bump up a selection of these to the 2nd tier, e.g., the ones to children, originally published in Letters to Children, and the ones to Mary Willis Shelburne, originally published as Letters to an American Lady; but to read the entire 3-volume set would be a 3rd tier project)
• Till We Have Faces (a difficult read, and perhaps I'd put it down in 4th tier, but since I reserved that tier for books I hadn't read and I have read this one, I left it here)

Fourth Tier - his academic works, his more obscure fiction, and his poetry (note: these are goals for me; I've not yet read any of them):

• A Preface to Paradise Lost
• Studies in Words
• An Experiment in Criticism
• The Allegory of Love
• The Discarded Image
• The Pilgrim's Regress
• The Dark Tower
• Boxen (not in this collection)
• Poems (not in this collection)
• Narrative Poems (not in this collection)
• Spirits in Bondage (not in this collection)

I wouldn't be inclined to try to read all of C.S. Lewis's works in one go. Dabble in them and as you get to know more about him over time, read some more. Read a biography of him somewhere along the line. And follow your interests, not necessarily what other people recommend to you. For example if you can't stand fantasy literature, don't read the Chronicles of Narnia early on. If you're a lit. crit. or philology nerd, read some of his academic works sooner.