mesmerizing (or mesmerising?)

I don't know which spelling looks correct? (or is correct? z looks better than s)

This book - is that word.

I don't quite know what it is about yet but it is filling me with unease as I read.
I thought it was a book about young adults looking fondly back at life at boarding school...

Wrong.

Dinner is going to be late because I got home from work and couldn't stop reading.
Oh well.