I'm not familiar with using the PRS+ firmware or how it allows extra fonts. But here is more detail on changing fonts per book.

To clarify, you can only change the system font styles of normal, italics, bold, and bold-italics. In other words, you can only have one custom @font-face per book. That font-face can contain one or more fonts as you can use the same font or up to four fonts to define the styles normal, italics, bold, and bold-italics, with each style using a single font file.

You can have as many fonts in the custom font folder as you desire and memory space allows, but you can only use one @font-face per book. Each book can be different though and use whatever @font-face you design. I created a pdf wherein I cover this in much more detail.

Is it complicated? Yep! Is it doable? Yep! And you only have to create the CSS once for a font-face and then save it as the default extra CSS for calibre to automatically add when it converts a book. You do have to look at the CSS for every book though to remove any font-family statements. So it is kluge but it works.