"The Dark Tower" was apparently envisioned as a sequel to "Out of the Silent Planet", (a main character is Ransom) written in 1938, Lewis wrote the notes for "Dark Tower" soon after, and then set them aside. It's about 77 pages;
For those of us who can't get enough of C. S. Lewis this little book of unpublished, and published-but-not-recently stories is a treasure trove.
