Translation:His fifth book disappointed me very much.
I don't think it's actually true that those are the only "true" equivalents of "really", since "really" gets used in a much broader rage of contexts. "Really" is the default intensifier in American English. It's not just for situations where you want to say "for real."
"Very" is not as frequently used and sounds more formal. And I can't remember the last time I said "this disappointed me a lot," I don't even think I'd ever say that. I'd only say "this really disappointed me."