Check out this thread:
https://www.mobileread.com/forums/sho....php?p=1892223
You have to name the font files exactly like the font name. If you open up a font file in windows you will see the font name at the top. So for Gentium Book Basic you need to name the files "Gentium Book Basic-Regular.ttf", "Gentium Book Basic-Bold.ttf" and so on.