My answer "Those books interest me very much" was marked wrong. The suggested answer Duo gave me was "Those books interest me SO very much." So very picky, my good friend Duo.
"I'm interested in a lot of those books" was marked incorrect, it should be accepted, reported
Your sentence would be "Me interesan muchos de aquellos libros."
Another translation for Duo's sentence (which is accepted) is "I am very interested in those books."
"Mucho" in Duo's sentence is not an adjective modifying books (because then it would be muchos), but rather an adverb modifying interest or interested.