for los libros de aventuras why is the a at the end of aventuras an a why not an o because of los
like for las hamburguesas there is an a in las and in hamburguesas and the end making it a feminine word
"aventura" is a noun (not an adjective). The prepositional phrase "de aventura(s)" can (in this case, does) function as an adjective but that does not alter the fact that "aventura" is a feminine noun.
Welcome to the forum. Libros is masculine thus the "los", aventura is feminine. I would give you more detailed grammar rules if I knew them but someone is sure to come along and help me out here. In the future please use correct capitalization and punctuation.