From there you can set the font used in all the panels by default.
If you want to only change the font for the Books panel, click "Set individual panel properties" and then remove the "Use Default" checkmark and set the font you want there.
The bottom-left of the main window has a zoom control:
www.swordsearcher.com
Finally, if you make changes to the font settings but the text in your Book panel doesn't change, you may be using a user-created module with hard-coded fonts in it. I don't recommend people make modules that way, but a module editor can hard-code fonts when they are making the module and if they did that, you'll have to resort to using zoom to increase the font size there.
MAN ! Do I love seeing members, "learning" ALL the tools of SS9.
After 12 years in it, I am still learning....
My "GO TO" is always my best friend; F1 and all the vids. Loll.
What a Blessing !