For example there is a series of books with 12 books.
I want to ask whether or not episode 12 is the hardest one using “the more , the more form”
What is appropriate to say it grammatically
"the later in the series the book is, the harder a level it is”
Literate English speakers don't write 'wanna'. You can pronounce it that way, but don't write it.
We also don't usually use 'episode' with books. Episode is normally for TV programs, and parts of a story.
There are a few ways to say it.
Perhaps also 'the higher the book number, the harder it is'.