I tried adding gamefont.css from the demo and declaring it in the index.html of my project, and add my custom font there. The font now appears on the book scenes, but only the second time they are loaded, for some reason ^^u
Ah, sorry! If anyone is having this issue there is a disclaimer about using rmmz-mainfont for MZ in this forum thread: https://forums.rpgmakerweb.com/index.php?threads/taa_bookmenu-v1-6-7.127880/