I enjoyed the books, but I enjoyed the original radio play[1] more. When it comes to books, I think Adams' best book is "Last Chance To See", which is apparently finally (well 2013) out in a new edition, so it's actually possible to get hold of):

https://www.amazon.com/Last-Chance-See-Douglas-Adams-ebook/d...

[1] http://www.amazon.com/Hitchhikers-Guide-Galaxy-Primary-Drama...