OK a lot of posts here, show that the history of H2G2 is poorly understood, it seems mostly by anyone who first encountered the 2005 film. So, to simplify, this was a radio show, then a book, then another radio series, then the TV series, then two more books, then three more radio series and another book, then a film.
So the TV series isn't an adaptation of the books, at most it and the first two radio series are substantially the same material, although it is a bit mixed up and it isn't entirely consistent (though it all was done by the author, so it is legitimate).
The radio shows used most of the same cast (except for Ford Prefect) and are the best version of the story (although it is a very long story, 15 hours or so). The TV series is a good rendering of the first two series and the first two books, inevitably it has some omissions.
The movie is pretty awful, and best avoided.
So the TV series isn't an adaptation of the books, at most it and the first two radio series are substantially the same material, although it is a bit mixed up and it isn't entirely consistent (though it all was done by the author, so it is legitimate).
The radio shows used most of the same cast (except for Ford Prefect) and are the best version of the story (although it is a very long story, 15 hours or so). The TV series is a good rendering of the first two series and the first two books, inevitably it has some omissions.
The movie is pretty awful, and best avoided.