There was the story "Young Zaphod plays it safe" that is included in the Ultimate Hitchhiker's Guide. I guess that I just figured that was the fifth book (or a series prequel anyway). I never read the salmon of doubt... I'll have to pick that one up at some point. Did you ever read the detective book/s he wrote? I almost picked one up years ago but I failed to at the time. I wonder if it is worth the time?