Romanian nouns change form depending on case (role in the sentence). The noun "carte" is here in the genitive case. All feminine nouns have a genitive form identical to the nominative plural one, so "cărți".
Unchiul meu este autorul unei cărți. - My uncle is the author of a book.
The second "i" marks the definite article:
Unchiul meu este autorul cărții. - My uncle is the author of the book.
So, my uncles is the author of the books is: Unchiul meu este autorul cartiile? Sau....?
Wow...this is confusing... Especially for a not native English speaker...(I haven't a clue what "genitive form = identical to the nominative plural fotm" means... hahaha