Look at the multiple attempts to adapt Hitchhiker's Guide. It's been done as a radio show, BBC mini-series and a Hollywood movie - all failing in various degrees to capture the humor and fun of the four books of the "trilogy".
There are five books in the "increasingly inaccurately named" trilogy. And the radio show was the original form, the books were in fact adaptations of that (the first two anyway). Everything else came after the books, true, but Douglas Adams worked on all of them.