Pat: maybe this will help
Here's another option for you: https://www.youtube.com/watch?v=XN64lF48fr4
Here's another idea, too! https://www.wikihow.com/Make-a-Hard-Cover-for-a-Paperback-Book
Thanks for inspirational question :) I will for sure do that too ...check that pin https://www.pinterest.com/pin/814940495045035650/
Here is a video to help you. https://youtu.be/9ToHqt2cS6Q
you would have to remove the paperback and re glue the seam to a hardcover
The cover needs to be stiff card board glue some on top of the pages of the cover and back
'Just curious as to what reason are you wanting to do this?
Is it for a particular project?
I have a collection of about 50 + well loved, well read paperbacks that are looking quite a bit shabby. I've been trying to replace them with good used hard covers a few at a time (I'm on a very limited fixed income) but some are very old and out of print. Like me in my dottering old age, I would like to spruce them up a bit.
My suggestion for preserving the outside of the books is to use library binding clear book tape to 'recover' the paperbacks.
Although it will result in a cover that is not exactly hard, this tape is much thicker and sturdier than clear packing tape, and is very hard to fold or rip. It is what school and public libraries use for repairing and reinforcing paperbacks.