TwistedBrush creates folders named Book1, Book2, etc under the TwistedBrush folder. On Windows Vista and Windows 7 depending on your UAC (User Account Control) these are stored elsewhere by the operating system and tagged as Compatibility files. Here's a link with further information on seeing this files:

http://www.pcmag.com/article2/0,2817,2124519,00.asp

On Windows Vista and Windows 7 you should find the files created from within TwistedBrush in the Virtual Store. For example this would be at

c:\Users\username\AppData\Local\VirtualStore\Program Files\Pixarra\TwistedBrush\

Windows Vista and Windows 7 place user files that are created in a secure location not within the real Program Files. Note in the path above replace username with whatever your user name is on your system.

Also see this item: Where are my TwistedBrush data files stored on Windows 7?