Interesting format - don't let the page count scare you, as the book pages are much smaller than usual for a hardcover. The reason is to enable the very neat poetry format of each page - some of the poems are certain shapes (that work with the poem/plot), most are not, but the poems must be on their individual pages. I really like the poem-within-a-poem of the transition from one twin's voice to the other.
I gave this three stars mostly for the uniqueness of the format, and it did keep me reading. However, the plot is on the low end of that 3 stars, since in a 564 page book I had figured out the big 'reveal' by about page 160.
Worth a YA's read, it is a good conversation-starter on so many topics - incest/abuse, sex, drugs, self-image, bulimia, cutting, etc.