Douglas Adams was a perfectionist - the main reason why the films, TV series, books and audio books are all different is him consistently trying to improve and change them all the time. Dont assume you know the story beyond the first bit if you have only known one format.
Its the classic plot of someone being uprooted without wanting to be, and then having no choice but to follow a slightly mad friend who wasnt who he thought he was at all, with no money, and even worse, no cups of tea to keep him going! These books make a satire of most of life - taking a good swipe at politics (if you dont vote for the lizards, then the wrong lizard might get in), religion (God giving away his existance and disappearing in a puff of logic) relationships (his perfect woman ran off at the party with a guy with two heads) friendship (no he wasnt from basingstoke after all, but a small planet somewhere in the vicinty of Beetlegeuse) and pretty much everything else between.
I know of very few people who havent loved the first of these books, once they put aside their prejudices towards fantasy, and then wanted to go straight onto the other books. This is an excellent way of buying them all economically, and dont worry, sadly there wont be any more books by him to extend the increasingly inaccurately named trilogy.