"Costigan's Needle" by Jerry Sohl. How do you survive in a new world without even the cloths on your back.
and
"The Ship That Sailed the Time Stream" by G.C. Edmondson. Romans and Vikings can be troublesome. Very funny.
These were part of the "golden age" of SciFi where the science took backstage to the people/action.
Both, ofcourse, are out of print but can still be found.
I have 2 versions of "The Ship That Sailed the Time Stream". 1965 and an updated one from 1978.
P.S. -
G. C. Edmondson was the working name of science fiction author Garry Edmonson (full name "José Mario Garry Ordoñez Edmondson y Cotton"). If you're lucky enough to read Spanish I'm sure he must be very good.