I bought this vintage 6 1/2 inch tall paperback at Uncle Hugo's Science Fiction Bookstore in Minneapolis MN USA on 12 August 2023. It is Ace G-631, printed in 1967, and I store it in a vinyl book bag. I have decided to carefully read this collectable book, and so put a bookcrossing bookmark with BCID in the vinyl bag, without attaching it to the book.

Neil R. Jones (1909-1988) was prolific science fiction writer of the pulp era. His most popular series of stories, published beginning in the 1930s, features Professor Jameson, a man of our own times who now explores the universe 40 million years in the future, as an immortal cyborg. The Jameson stories are known to have inspired Isaac Asimov as a child, and were known to other writers of the later golden age. In all, there were twenty-three Professor Jameson stories published between 1931 and 1951. A few more were published posthumously, or never published. In 1967, Ace Books re-published many of the stories in the form of five short books, each collecting several stories, in order. Collection #2, “The Sunless World”, contains the second three stories:

Into the Hydrosphere, Amazing Stories, October 1933.
Time's Mausoleum, Amazing Stories, December 1933.
The Sunless World, Amazing Stories, December 1934.

By this point, Professor Jamison is a well-established member of the cyborg Zorome crew. These three adventures involve the crew on its way to Zor. They stop first a world seemingly made up entirely of water, then time-travel back through the geological history of the Earth, and then a world in intergalactic space associated with no sun. Along the way, they battle with intelligent deep-sea frogs, and headless brutes. The science of these stories seems preposterous to modern sensibilities, but it is important to remember that the human experience of space travel did not start until decades later.