I’m confident that the longest book I’ve ever read is Harry Potter and the Order of the Phoenix.
In fact it was so long I’m sure that the first time I read it I actually got a little bit bored. Of course I hate myself for saying that but it was 11 years ago* and I was 9 and obviously ignorant. I appreciate it now but it’d still take me a long time to get through the first pre-Hogwarts chunk of the book.
*I had to look that up. I was in shock for about 5 minutes. Where did all that time go?!!!