Hi,
i used book view only to have a preview larger than i have the preview normally. I have Book browser, code view and preview, and code view is taking trhe most space. So theres only little space left for preview window. Of course you can drag the divider between the windows to resize them, but i can understand the whish of toggling between code view and preview fast, being in the center of the screen and consuming more space.
If there would be a button to switch postiob from code view and preview window would be really fine for some users.
Personally i prefer that resources are spent for other issues