Build your 6 layers (all squares) in Photoshop. One layer is your cover, another layer is thin lines to look like pages, etc...
When you've finished building out your square cube, just squash down the Group's Y scale (to 20%ish to flatten it) and Z scale (to 70%ish to make it more rectangular like a book). You can always add other details like Text to the Group later by matching up the Text Layer's rotation/position with the Cube layer's rotation/positions.
"The deepest blues are black" - Foo Fighters
(this doesn't help me when I'm chroma keying!)