Here's a little perl script I knocked together for converting Gutenberg HTML books. It produces a HTML format suitable for the
Librie Toolbar (which creates BBEB books).
You additionally need
wget and
unzip installed to get it to work. It should work fine on MacOSX and Linux. For Windows I use Cygwin (cygwin.com).
To use it simply pass the location of the ZIPed HTML file on the Gutenberg page for the Book you're interested in, for example:
guthtml.pl http://www.gutenberg.org/files/17290/17290-h.zip
Then you just open the "new.htm" file in Internet Explorer and use the Librie Toolbar to create BBeBs

I find the Librie Toolbar at the moment produces the best eBooks as it has the cleanest fonts and these are the fastest on the Reader. It doesn't produce perfect conversions but until a proper tool comes out I use this for now.
To convert text based versions of Gutenberg I simply use GutenMark and convert the produced HTML file with Librie Toolbar.
The script can also call
htmldoc if you want to create PDFs for the Reader, simply uncomment the last few lines in the script.
If anyone knows how to get page breaks to work in Librie Toolbar please let me know.