Into the Wardrobe

Hubby and I went out of town for our anniversary Thursday,and stayed at a rather bookish inn; I was delighted to find a copy of Lewis' Narrative Poems in the room, which I'd never read. I got through Launcelot and part of Dymer.
Something in Walter Hooper's preface to the edition surprised me, though. He makes a comment to the effect that Lewis composed Surprised By Joy in poetry before writing it in prose. What is Hooper referring to? I'm sorry I don't have the direct quote -- unfortunately the bookstore below didn't have a copy I could purchase (all the better for John though, so I can buy the book thru this website).
Does anyone know what the heck Hooper was talking about?