Short answers, pulled from the story.
The Hitchhiker's Guide to the Galaxy began as a radio sitcom broadcast on BBC Radio 4 in 1978, created by Douglas Adams. It started from a proposal called "The Ends of the Earth" and was adapted into novels, television, film, and other formats over the following decades.
As of 2005, the first novel had sold over 14 million copies. It reached number one on the UK book charts in its second week and sold over 250,000 copies within three months of its 1979 release.
Eoin Colfer, author of Artemis Fowl, wrote the sixth book, And Another Thing, published in October 2009. He was commissioned with the support of Jane Belson, Douglas Adams's widow, following Adams's death on the 11th of May 2001.
Peter Jones voiced the Guide, described as "The Book", in the first two radio series and the television version. He was cast after a three-month search and after at least three actors, including Michael Palin, turned down the role. William Franklyn took over for the third, fourth, and fifth radio series, and Stephen Fry voiced the Guide in the 2005 film.
In the series, 42 is the answer given by the supercomputer Deep Thought to the Ultimate Question of Life, the Universe, and Everything, after eons of calculation. The joke became a widespread cultural reference, appearing in works including Doctor Who, Lost, Star Trek, and The X-Files.
Towel Day is a fan-created event celebrated on the 25th of May each year, in which participants carry a towel throughout the day. It references the importance of towels as described in The Hitchhiker's Guide to the Galaxy and was started in 2001, two weeks after Douglas Adams's death.