Two books of scripts, actually: Big Mystery Joke Book and Big Book of Plays.
I have to agree that the first four albums are the core of their work. Bits and pieces of later work are good but nothing really compares to that four year bit of brilliance.
I still remember the day in college when my roommate and I were listening to Don’t Crush That Dwarf… for the umpteenth time, came to the bit about having “an argument with my co-pilot” and shouted out simultaneously, “God is My Co-Pilot!” Layers under layers. There’s still a need for the Annotated Firesign Theatre.