EDIT2: Ignore my Edit, below, saying to ignore my OP
. That Novik book actually has included font files for the fonts set up in their advertising pages. But, I found a Brandon Sanderson book ("Yumi and the Nightmare Painter", published by Dragonsteel Entertainment (which appears to be the company he set up to publish his works -- so, I guess it is self-published) that's "using" the Palatino font without a font file and without an at-rule. So, as you said, this could be a misunderstanding on self-published books. Though Brandon Sanderson doesn't sound like a person who'd make that kind of mistake.
EDIT1: Ignore my OP, below. I think I figured out what's happening. It looks like whoever put the books together is instantiating the fonts with those @font-face commands in <style></style> blocks on the advertising xhtml pages which I'm deleting (instead of in a separate stylesheet). And, I think they're then using those fonts in the actual stylesheets. That's going to make my examination of the use of those fonts a bit more interesting. /EDIT.
OP: I guess that's possible with the book I mentioned. That's one of Gene Doucette's "Fixer" series. And, those appear to be self-published. But, I'm looking at another book right now ("A Deadly Education" by Naomi Novik) and it appears to be published by Del Rey. It has 6 un-instantiated fonts in it. Oops. See above.