Why don't you need "la" in front of livre? I was under the impression that adding of in front of something turned it to the generic version like "Je dois acheter du pain" for "I must buy bread" not "I must buy THE bread", which Duolingo will certainly count you wrong on if you include "le". Based on what we previously learned I expected this to be "My uncle is an author of books" or "A book author" ...
"un livre" is masculine to mean a book, so you cannot have a feminine article in front of a masculine noun.
"du" is not only a partitive article, it is used in number of cases where the preposition "de" and the definite article "le" are both needed.
"du" is the contraction of "de+le", whether or not this is the translation for "of the" or "some" or "about the".