Braden has read 35 books.
12 of those books he read before joining the book club.
If we subtract 12 from 35, we will get the number of books that Braden has read in the book club:
He has read 23 books in the book club. Since they read 1 book per month in the club, that means that he has been in the club for 23 months.
Answer: 23 months