I love reading book series... getting totally absorbed in the characters and plots for thousands and thousands of pages. My all time favorite series is Stephen King's Dark Tower series.
Sai King started writing these 7 books with The Gunslinger a couple of decades ago & created, in my opinion, a masterpiece.
Far from being a horror story, as King is always touted for, the Dark Tower books follow Roland of Gilead, the last Gunslinger or knight, through his quest to reach the Dark Tower - the central point holding together the universe but at risk of destruction. Roland's quest lasts hundreds of years through various parallel worlds, with time changing and warping taking him further from his past as the world has moved on.
Time changes, worlds meet and magic happens in this great epic.