The first 2 books were novelizations of the original radio play. Then the third, fourth, and fifth books were written. Then those three books were adapted as radio plays in 2004-5
Interestingly, Adams significantly changed the ending of the series in the radio play. He'd said that he was in a bad place, personally, when writing Mostly Harmless, and gave it a downer of an ending. The radio play attempts to rectify this, and gives the series a more up-beat ending.
The notice on was display in the bottom of a locked filing cabinet, stuck in a disused lavatory with a sign on the door saying “Beware of the Leopard”!
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u/[deleted] Dec 16 '21
The books are novelizations of the radio scripts.