I largely agree with glowing fish. I obsessed over The Magician's Nephew like no other book when I read it in 3rd grade. Much more so than the other Narnia books. It appears that The Wood Between the Worlds does indeed represent a profound concept of a Multiverse, and possibly derives from a real mystical experience. Quoting CSL:

"as in the sinless world beyond the horrors of animal and human life;
in the behaviour of stars and trees and water, in sunrise and
wind. may there be here (in my heart) a like beauty"

the novel "The Wood Beyond the World" may have offered inspiration. he expresses a lot of interest and thought about mystic travel. This quote from Letters to Malcolm:

"I do not at all regard mystical experience as illusion. I think it shows
that there is a way to go, before death, out of what may be called
"this world"--out of the stage set."

"The lawfulness, safety, and utility of the mystical voyage depends not
at all on its being mystical — that is, on its being a departure — but
on the motives, skill, and constancy of the voyager, and on the grace
of God."

Now, glowing fish mentioned that this type of experience is not atypical of what one might experience on drugs. I have been told by a friend that after smoking Salvia Divinorum he had felt like he was in The Wood linking to all worlds from the Narnia book. My best efforts have been unsuccessful to recreate this experience myself. My experimentation with Ketamine was far more enlightening. Take this last quote from Lewis himself:

"I shouldn't be at all disturbed if it could be shown that a
diabolical mysticism, or drugs, produced experiences indistinguishable
(by introspection) from those of the great mystics. Departures are all
alike; it is the landfall that crowns the voyage."

Lewis goes on a bit about his desire to look behind the scenes of the stage set of our world. Whether or not "The Wood Between the Worlds" stems
from a real experience or merely a fictional construct, it has left a mark on my view of reality.