Lots of stuff to learn for this one.
(
A lot of good info is in the MR Wiki)
When you added a font file using the File menu in the editor, it supplied some code on (on the clipboard), that you need to PASTE into your LINKED CSS.
This may-will need adjusting (path in the book, Family name)
The code for where it gets used, needs MOST of that same info in the font use call: Family, weight, style
BUT
You can't just embed fonts into books
for sale/distribution. You need PERMISSION ($$) from the font foundry in many cases.
(adding for personal use is a breeze. You need waaaay more experience for commercial use. Not all devices allow publisher font usage)