Favourite book is At Swim, Two Boys - but it’s a challenging book and therefore difficult to binge. If you mean something I can read a trillion times, then Order of the Phoenix.

HP single-handedly created the YA genre if I am not mistaken. Obviously there were books for teens (which is what YA is), but YA as a separate literature/marketing category did not exist. Now YA is the most lucrative genre. And pretty much all the successful YA books (Twilight, Hunger Games etc) followed the same style as HP: being a series instead of a single book, online fandom culture, big budget movies etc.