In the beginning, The Hitchhiker's Giude to the Galaxy was written. This made a lot of people happy and is generally regarded as a good move.
Some time later, it was followed up (by a sequel). This also made a few people ("The people...the things..." "The things are also people," hissed Ford. "The people...the...other people...") very pleased. I am among them. DNA is an excellent writer and this book is perfect alone, after its predecessor, or with a Pan Galactic Gargle Blaster. Brilliant satire, wonderful characters, and the depressed droning of our favourite Paranoid Android all contribute beautifully to a work of sheer unadultered weirdness. I'd reccomend reading HHGG first to all newcomers to the HHGG trilogy, but if you've already read the first in the series the best way to follow it up is by reading the second. Or by stopping for lunch at Milliways--The Restaurant at the End of the Universe! (But don't forget your towel!)