correction (kind of)
officially, there are five books in the trilogy. douglas adams hadn't necessarily meant for "the salmon of doubt" to be published and included in the series... it was published post-humously, arranged and finished by his estate. apparently he hadn't done even 70% of the writing. i haven't read it but my girlfriend said it was pretty good. i guess i'm skeptical because i resent posthumous publications generally... seems like nothing more than a profit machine to me... but i'm a bitter old coot!