Is there a difference in content between the two versions of The Hitch hiker's Guide to the Galaxy? I've only ever listened to the radio show, and I was wondering if a) there was anything to reading the books, and b) if reading the sixth book was going to leave me having missed something.
The first two seasons - ie the originals, which Adams was actually involved with - are quite different to the books. The first season actually came first of all, and includes parts - although nowhere near all - of what became both the first and second books. The second season only has occasional similarities to the second book, with large amounts of completely different material (Lintilla and Poodoo appear nowhere in the books, for example). Adams was actually proud of the fact that the books and radio series totally contradicted each other.
Those two series were made in the 70s/80s, but after that no more were made during Adams' lifetime. Much, much later - in the mid 2000s - the BBC decided to commission adaptations of the three other books. Naturally, since Adams was no longer around, the adaptations were much less free - although some material was new, the bulk is the same. This is why there's a total jar at the beginning of the third radio series - to get the story back on track with the books, they had to dismiss all of the diverging continuity as a dream.
It's definitely worth reading the books. You won't miss much in terms of continuity if you don't, but they're hilariously funny in themselves, and of course contain a lot of both story and humour that didn't make it into the radio versions.
I think it's a difficult question to answer, simply because there are so many differences. Perhaps I'll do best to quote Douglas Adams' own recollections, made in the introduction to The Ultimate Hitchhiker's Guide to the Galaxy, a collected works volume published in 1995.
Under the heading Some unhelpful remarks from the author, Douglas confides that -
Writing episodically meant that when I finished one episode I had no idea about what the next one would contain. When, in the twists and turns of the plot, some event suddenly seemed to illuminate things that had gone before, I was as surprised as anyone else.
He goes on to add -
People I talked to seemed to like Marvin the Paranoid Android, whom I had written in as a one-scene joke...
and in a radio interview on the BBC's website from this period (The Doctor and Douglas), recorded when he was script editor of the Doctor Who tv series, he describes with some candour how writing Hitchhiker's was basically a process of just sitting at the typewriter trying to think up a succession of jokes, and occasionally having one of his characters come out with a one-line remark to get him out of a plot-hole he'd dug himself into.
He recalled that the first novel only dealt with part of the original radio series -- due to his publisher turning up on his doorstep and seizing everything he'd written up to that point, after he missed the 10th deadline (yet had still only novelised the first 4 scripts):
[It] was a substantially expanded version of the first four episodes of the radio series, in which some of the characters behaved in entirely different ways and others behaved in exactly the same ways but for entirely different reasons, which amounts to the same thing but saves rewriting the dialogue.
And as to the 2nd novel:
In the fall of 1980, the second Hitchhiker book was published in England ... It was a very substantially reworked, reedited and contracted version of episodes 7, 8, 9, 10, 11, 12, 5 and 6 (in that order) of the radio series... [and] was called “The Restaurant at the End of the Universe”, because it included the material from radio episode 5 ... which was set in a restaurant called Milliways, otherwise known as the Restaurant at the End of the Universe.
And, behold! There was also the influence of the television series:
Meanwhile, a series of six television episodes of “The Hitchhiker’s Guide to the Galaxy” was made by the BBC and broadcast in January 1981. This was based, more or less, on the first six episodes of the radio series. In other words, it incorporated most of the book “The Hitchhiker’s Guide to the Galaxy” and the second half of the book “The Restaurant at the End of the Universe”. Therefore, though it followed the basic structure of the radio series, it incorporated revisions from the books, which didn’t.
But that was as nothing. For then this happened:
In the summer of 1982, a third Hitchhiker book was published simultaneously in England and the United States, called “Life, the Universe and Everything”. This was not based on anything that had already been heard or seen on radio or television. In fact it flatly contradicted episodes 7, 8, 9, 10, 11 and 12 of the radio series. These episodes of “The Hitchhiker’s Guide to the Galaxy,” you will remember, had already been incorporated in revised form in the book called “The Restaurant at the End of the Universe”.
I wrote a fourth and last (sic) book in the trilogy, “So Long, and Thanks for All the Fish”. This was published in Britain and the USA in the fall of 1984 and it effectively contradicted everything to date, up to and including itself.
So there you have it. A lucid and coherent tale if I ever heard one. As told to Oolon Coluphid by someone who might have been expected to know better...
As near as I can figure, the events of 'Fit the eleventh' are almost non-existent in the books: we do not meet Lintilla or her clones, nor do we hear of the dire economic situation that was caused by an excessive shoe market on the planet, and the bird-species' subsequent evolution to avoid the ground- and shoes- at all costs.
Arthur's argument with the nutrimatic drink dispenser is still there, but not the huge statue that was built in honour of the event.
The way that the crew get to Milliways is different, also: in the radio series it was due to an exploding computer terminal, thanks to Shooty and BangBang's eagerness to kill the crew; whereas in the end of the Hitchhikers Guide To The Galaxy book the crew choose to go there- admittedly whilst escaping from the Vogons.
I would very much suggest you read the books if you have the chance: in my opinion, Mostly Harmless was when Adams' writing was at its best- although some fans may claim it's depressing.