I’m glad you like. The number of items used is controlled by the general WP ‘Setting -> Reading’. By increasing ‘Syndication feeds show the most recent’ number the plugin will pull more posts into the PDF. Word of warning though, set it too high and the pdf engine will run out of memory.
There seem to be more WP plugins for making books out of blog posts like http://anthologize.org/
Might be worth checking out