PortaBook is an evolution of an applcation called PSPBook by Robin Ward, which is no longer being maintained.
The PortaBook source code is available anonymously from http://opensource.creative.com/repos/portabook.
From the command line, the following command should retrieve the source code:
svn checkout http://opensource.creative.com/repos/portabook/trunk portabook
A binary disk image is also available for download -- PortaBook_1_0_3.dmg.