What a surprise! We just got Road Tapes #3 and now there are two more CD's on there way.
The Crux Of The Biscuit seems to be the most interesting one. It was created in conjunction with the 40th anniversary of Zappa's highly rated 1974 album Apostrophe ('). It contains rare alternate mixes, live performances & studio session outtakes.
The other one is called Frank Zappa For President. Seems to include some 80's material, but maybe other stuff as well. You never know what is behind those titles.
Track lists to both albums here.