Happy birthday! And please correct this oversight immediately! Don't bother finishing the movie; despite Douglas Adams' personal involvement in it, it's not that good IMO.

I've read all of Adams' books, and had the fortune to see him in person twice on book signing tours. One of the tours was for "Mostly Harmless", the fifth book in the Hitchhiker "trilogy", which is considered the weakest book of the series. All the others are great though, as are the Dirk Gently books.

The other book tour was for the excellent non-fiction "Last Chance to See", about his travels around the world to observe endangered animals. His friend Stephen Fry carried on this work with zoologist Mark Carwardine in a documentary series for the BBC, which included this absolute riot of an animal encounter: https://www.youtube.com/watch?v=9T1vfsHYiKY