The concept of “genre” can be a fairly slippery one, but allow me to propose that genres of fiction can be broken down into four categories:
(1) Setting
(2) Plot
(3) Tone
(4) Target Audience
Which can roughly be explained like this:
(1) If a book is set in the 14th century, its historical fiction. If a book is set on Mars, its science fiction. If a book is set in a magical fairy kingdom, its fantasy. And so forth.
(2) If a book’s plot is significantly based around solving a crime or puzzle, then it’s a mystery. If a book’s plot is significantly based around two people falling in love, then it’s a romance. And so forth.
(3) If a book is supposed to make people laugh, it’s a comedy. And so forth. (This one is a bit harder to get your thumb on.)
(4) If a book is meant to be read by children, it’s a children’s book. If it’s meant to be read by teens, it’s a young adult novel. If it’s not meant to be read by people younger than 18ish, then it’s a mature novel.
You can freely mix-and-match between the different types of genres (a historical children’s mystery comedy). Mix-and-matching with another genre of the same type is bit trickier. I think the only genre which is truly exclusive is setting.
I have a conundrum. If an author takes and ties multiple books using different genres of type (1) together, do we need to separate out the books? all of the tie ins for Shannara strikes me as an example of this as does Mercedes Lackey’s Valdemar works.
Usually ML starts off a trilogy with a YA book and then the character finishes school, becomes an adult, then have very MA/racy adventures thereafter.
Most of Shannara is Fantasy until you get into the post Apoc prequels…