There are many ways to design a book and many opinions on what looks best and what’s right and wrong. I always try to keep things as simple as possible and work to make the interior of the book look good while still being easy to read.
When I published my first book I had little experience in design so I decided the best way to learn was to go to a local bookstore and study books on the shelf. I looked at many genres and learned that it was difficult to find two book alike. Design seemed to be a matter of choice and opinion. I advanced my research to finding the designs that I really liked and then I studied how I could improve them and make them mine.
One of the things I noticed and found troubling was that many books had quite narrow margins next to the spine of the book. This made the book difficult to read plus you almost had to damage the spine to read comfortably. The second thing I noticed was that some fonts were much too small. The only reason I see for the tiny font is reducing the number of pages. Within reason this may work but if the font gets too small to read comfortably it’s not a good idea.
I even saw several books where the design involved a very narrow margin at the spine but a large and generous margin at the outside edge. This may be considered an attractive design but I didn’t care for it and have never used it on any of my books.
My method is to allow adequate white space, a large enough font, and sufficient room at the spine to make the book comfortable to read.
As to fonts, there are plenty of design articles to provide information for a good choice. If in doubt I would format several pages with different fonts and then print them and select the one that seems best.
There is much more to learn about design and you might even consider hiring a book designer if you are unsure how to proceed.