Perhaps you can help this friend of mine, who messaged me:
Here is the plot of the Sci-Fi novel that is my obsession (I am thinking about it most of the time). The protagonist is traveling through space when his ship somehow breaks down near a planet that is mysteriously only 1/2 visible. It seems that some kind of force field has cleft the planet into two hemispheres, and rendered one invisible. He lands his crippled craft near the edge of the force field (on the visible side) and sees a shimmering wall that goes as high as the eye can see. That is the force that divides the planet. He finds himself in a rural area on a world where normal physical laws do not apply. He is trying to find a means to repair his spaceship, but the people and technology of the region are primitive. Trying to find some way to get help, he gets passage to a distant large city on a river barge that is involved in an expedition hunting for strange creatures. He pays for his passage by signing on as a crew member. Among the prey are large froglike things called batrachians. The hunters catch several of these (with suitable excitement) and then go for the prize game, a large cat-like animal called a _. They find one, and net it, but as the net is swinging upward it swipes our hero with its paw, scratching his cheek (I think). Somehow this critter and our hero get bonded. I think the beast has some ESP, because when it gets loose, it continues to follow him through his adventures, and saves his bacon a few times.
He arrives at the large city, and is amazed at the primitiveness of the technologies used by the citizens. He thinks “With my advanced engineering skills, I can really help these people.” So, against the protests of the people, he builds a windmill out of metal, to draw water in place of a well with wooden buckets. To his frustration, every time he starts the vanes, the thing collapses. The people tell him he needs the right magic spell, and that nothing made of metal functions on this planet. He learns that entire culture is based on ‘natural’ materials (fibers, wood, rubber-like sap and so forth) and all is held together by magic spells. He is not popular with the local wizards, and has some adventures avoiding their wrath.
He finds himself on a large sailing ship, captained by a pirate named ‘Sarn, the Melon Bellied’. Sarn pretends to be his friend, but plans to sell him into slavery when they reach the next port. The cat-like creature saves him, they swim to shore, and the journey continues overland to more amazing adventures. They (he and the critter) become companions (like Beastmaster) and occasionally see visions, as the time/space fabric on this half of the planet is strained and they get a peek at some events that are happening in another place or at another time.
Ultimately, he cannot find the help he needs to fix his spaceship and exit the planet. He and the critter arrive back at the force field wall, and it turns out that the critter has the magic mojo to cause the wall to open. He and the animal cross into the far side, where new adventures await. The end.
This is a little hazy, as I read (and reread) this book about 40 years ago (and I think I got it second hand when I first read it.). I may even have themes from more than one book mixed into the tale. However, the above is about all I can recall right now.
If anything in the sketchy plot triggers a search term in your mind, or gives a hint that might lead us eventually to the book, I will be very, very grateful.
(Note: It's not The Janus Stone by Andre Norton or Batrachian by Keith Laumer)