It's not like Douglas Adams intended it that way!
He originally was going to write one novelization based on the radio series. But, he got half-way through the story, when he decided to cut it short, and make two books. And, that was supposed to be the end of that.
Then the books got really, really popular. So popular, that Adams was pressured to write a 3rd book. He did so by ripping off an idea he originally conceived for a Dr. Who episode, that did not get completed (until the 1990s, anyway). And that, as they say, was supposed to be the end of that!
Then the books got even more popular, so he wrote a forth one, as well, to end the series. But, he neglected to make it impossible to write any more of them.
Then, several decades later, he wrote a fifth, because he felt the need to end the series in a manner that would make it impossible to write any more of them!
Then, while writing a new Dirk Gently novel, he decided to change it into a sixth HHGG book. But, he died before he could complete it. Some of the scraps of this uncompleted masterpiece are published in The Salmon of Doubt.
That is the story, in a nutshell. I am leaving out lots of stuff: The computer game, the TV series, the other books he had written, his propensity to miss deadlines, etc.
The novels are very good. My favorites, of course. The radio series, both old and new, is relatively good. But, the movie leaves much to be desired.
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