A-Z Challenge: I is for "it" factor

Every now and then, I come across a book I literally cannot put down. I forgo meals, sleep, just to find out what happens next. And once the journey has ended and we've gone our separate ways, I might even go through a little post-book grief.

I imagine, for any author, that's the goal: to create a work with an "it" factor. But what is "it" and is it the same for most (or all) readers? Of course, if it was as simple as dropping in the "it" factor/formula/etc., then every book would have it. That sounds like a lovely idea: every book a masterpiece. But if it was that simple, then the treasured affect would diminish. The hunt would be null and void. I like that the "it" factor is a bit of a mystery.

For me, the "it" factor in a book is simple: I have to believe, with all my heart, in the main character. I don't have to always agree with him/her, but I do have to feel like, in reading his/her voice, he/she is telling me their story exactly as they know it to be true. When a writer does this, I'll follow that character anywhere: to spaceships, magical woods, dragon caves, political arenas, underwater, etc. The voice of the writer is absolutely the "it" factor for me. And it's what I tried very hard to focus on in Moonlit.

A few of my "it" factor books:
Bloodroot, Amy Greene
Hunger Games, Suzanne Collins (book one only)
The Black Stallion Series, Walter Farley
Kite Runner, Khaled Hosseini

What is your "it" factor? What are your treasured books that have it?

See you tomorrow. I'm tell you the whole story behind my pen name, Jadie Jones.