I think the biggest book I've ever read is Stephen King's "The Stand", and I've read it more than once. Read as long as you're interested, then put the bookmark in and give it a rest until you find yourself interested again. Don't force yourself to push onward, take it at your own natural pace.

I'm currently reading the quran for the first time. The second surah was very long, it took me multiple sessions to get through. There are over 100 surah. It will take me months to finish. It's very repetitive. But I want to know what's in it because many people believe it and are even willing to kill and die for it. So I try to process each statement as I go, and I don't worry so much about when I'll get to the end. It will happen eventually if I keep reading. It's also not the only thing I'm reading, I switch to whatever I feel like at the time.