I read about half of Robert Jordan’s Eye of the World (Book 1 of Wheel of Time) in about 9 hours. I was around 14 and I went to bed at 8pm and read straight through until 5am. I remember this clearly because Dad woke me up 30 minutes later to force me to go fishing for the day.
A Portrait of the Artist as a Young Man by James Joyce. I had to read it for uni. Hardest book I’ve ever read