hide invisible flippy book overflow

This fixes #322. The big negative margin on the book cover content is making several browsers expand to render all 9999 of those invisible pixels. By giving `#book-container` an `overflow: hidden` rule, this seems to fix the problem.

I had some difficulties with my dev environment and ended up rebuilding all of the assets per @bry4n's advice (thanks!), hence the larger diff.

/cc @ben @matthewmccullough