Format RB is extension format for type of e-books called Rocket eBook by company called NuvoMedia. Later on it was used in devace named REB 1100 created by Gemstar. Its advantages are use of HTML, pictures and info files. That contains all information about name, author, copyright, etc.
There´s a big cluster of programs that allow you to open these files saved in .rb format. First of all there is application RocketLibrarian developed by NuvoMedia. After that there is eBook Librarian by company Gemstar. Then there are programs like GEB Librarian or Book Designer, both of them are for Windows. At least but not last there is rbmake for Unix.