I wrote this software back when I was heavily into the Rocket eBook and the REB1100. It takes HTML and/or text files and turns them into an ebook (in the ".rb" format). The utility is called rbmake, and it's a command-line tool, which means that it is targetted at a pretty "techie" audience. At least two online booksellers were using it to create their non-encrypted books, so it was pretty useful back in its prime (the book sellers were FictionWise and Baen Books).
If you're interested in how unencrypted .rb files are put together, see this page that describes how the .rb file is put together.
I also have some older scripts for modifying Baen WebScription eBooks. These scripts aren't really needed these days, though, so they only really have historical value (if any at all).