My monitor resolution is 1280x1024 (I know, I know) and Minecraft's drawable area is 1280x962. Using v328.

With the normal book size:

• GUI scale Small: http://i.imgur.com/eyHc2Yw.png
• GUI scale Normal: 👌
• GUI scale Large: http://i.imgur.com/cG8szxV.png
• GUI scale Auto: http://i.imgur.com/qDPEE1q.png

With the second book size (button clicked once)

• GUI scale Normal: http://i.imgur.com/SHj6oS3.png

You get the idea. It seems that certain components render at the book's scale (previous/next/back buttons, menu items, the main menu) and certain things render at the GUI scale (book text and images, search bar, "shift" text)