Yeah, the Radio Play was the original format of Hitchikers, the books came after.
There's the full cast BBC Radio Plays which I adore, and then there's a sort of half way house where you have a full cast reading of an abridged (or unabridged) version of the novel, with sound effects sometimes. My least favourite is unabridged readings, with the sole exception of Martin Shaw's unabridged reading of The Silmarillion, which I believe is the best way to approach that particular book. Reading a book is a very different experience to listening to one read, and in those cases I prefer an abridged reading, like Tony Robinson's versions of the Pratchett novels.
I have a pretty large collection of all types, from the full cast BBC LotR, to the small cast Big Finish Audio Productions of Doctor Who, to the Philip Pullman novels with full cast (both radio play and unabridged cast reading) to the Terry Pratchett ones. I'm happy to share if anyone is interested. I've a pretty extensive collection.
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These are my friends, see how they glisten...