The Hitch Hiker's Guides: A Trilogy in Four Parts by Douglas Adams (Paperback)

Books in the Series (Trilogy) are as follows:

• The Hitchhiker's Guide to the Galaxy

• The Restaurant at the End of the Universe

• Life, the Universe and Everything

• So Long, and Thanks for All the Fish

• Mostly Harmless

• And Another Thing...

Two omnibus editions were created by Douglas Adams to combine the Hitchhiker series novels and to "set the record straight". The stories came in so many different formats that Adams stated that every time he told it he would contradict himself. Therefore, he stated in the introduction of 
The More Than Complete Hitchhiker's Guide that "anything I put down wrong here is, as far as I'm concerned, wrong for good." The two omnibus editions were The More Than Complete Hitchhiker's Guide, Complete and Unabridged (published in 1987) and The Ultimate Hitchhiker's Guide, Complete and Unabridged (published in 1997).