I see that the GS-F&FW website lists the Hitchhiker's Guide novelizations and the two Dirk Gently books, but it does not list the work that made Adams famous in the first place, and made it possible for him to publish all of those books. I refer of course to the original BBC radio series "The Hitchhiker's Guide to the Galaxy", which aired 6 episodes in 1978 and another 6 episodes in 1980. The novels came later, and diverge significantly from the plot lines of the radio play.
The complete scripts, somewhat annotated, are available as The Original Hitchhiker Radio Scripts, by Douglas Adams, (c) 1985 Harmony Books, New York. My edition features an Introduction by Geoffrey Perkins (the producer of the radio series) and a preface entitled "Where do you get all your ideas from?" by Douglas Adams.
Even if you have all but memorized the radio shows (as I had, at least at one point in my life), it's worth getting a copy of the scripts just to be able to read the author's special effects descriptions and the footnotes to the various episodes. And, while the original radio series is perhaps not a work of 'literature', one could argue that the radio scripts are, in the same way that the text of a play is literature. This is not in any sense "the same" as the novelization, and deserves a place of its own in the listings.
Or so I assert.