Just finished this book.
Enjoyed it.

But the description of Heaven (or the foothills of Heaven) and the ghosts interaction with it reminded me of a sci fi book I had read when I was a kid.
The book was called "Singularity" by William Sleator.
From what I remember, two boys stumble across some shed that takes them to other dimensions. I don't recall too much from the book except that one of the universes they travel to everything looks like earth, but the grass blades, water and everything is too solid for the boy to interact with, without hurting himself.

Just like the description of the foothills of Heaven and the ghosts inability to interact in a meaningful way... and their tendency to get hurt because everything is so much more solid than them.

Hmmmm...
Maybe it was "The Boy Who Reversed Himself"...not "Singularity".

GREAT CESAR'S GHOST!!

SINGULARITY is the one with the shed. I think my daughter tried to get me to read it when she was around 13-14. I couldn't finish it. But I don't recall the similarity to Lewis' concept. OTOH, I couldn't finish it.

GKC