It's a known bug for KF8 books read on PW1 and PW2. If book uses some embedded fonts only for things like headings
and font-family is
not declared for body (nor paragraph) CSS class, PW1 and PW2 fallback to Helvetica/sans serif instead to Caecilia/default serif font.
So you must edit ebook file to add
body {font-family: serif;}, that fixes the problem. That was font related bug #1
Font related bug #2 that also relates to body and p tag CSS code is the one where the book gets stuck with Caecilia font and changing to another built-in font in Aa menu does nothing. That is what you get when there is no font-family for body and p tags CSS contains crap like "Times" (or any other referenced but not embedded font family) or even just font-family: serif;.
Conclusion: always define book's default font in body tag
Maybe it would be possible to use Font Override part of @NiLuJe's font hack to temporarily override sans serif font with a serif one, while you are reading problematic book, and that way to avoid having to edit the ebook file itself. But you should probably just stick to editing files. It easier and safer option.
EDIT: I'm to slow.