Short answers, pulled from the story.
Durarara!! was written by Ryohgo Narita and illustrated by Suzuhito Yasuda. The first novel was released in April 2004 by ASCII Media Works under their Dengeki Bunko imprint. The series ran for thirteen volumes, concluding in January 2014.
Durarara!! follows Mikado Ryugamine, a teenager who moves to the Ikebukuro district to attend Raira Academy, and becomes entangled with a cast of characters including a headless dullahan courier named Celty Sturluson, a superhumanly strong man named Shizuo Heiwajima, and an anonymous internet gang called the Dollars. The story rotates among roughly eleven character perspectives, each revealing different pieces of a larger plot.
According to author Ryohgo Narita, the title means absolutely nothing. He invented it on the spot when his editor called asking for a title while he was editing the finished manuscript. His editor approved it because he liked ambiguous titles.
The first Durarara!! anime season began airing on the 8th of January 2010, produced by Brain's Base, on MBS, TBS, and CBC. It ran for 24 episodes through June 2010 and adapted the first three light novels.
By March 2018, the Durarara!! light novel series had over 5.6 million copies in circulation, according to available sales data.
Yes, a sequel light novel series titled Durarara!! SH, set two years after the original, began in 2014. Only four volumes have been released, with the series stalled due to Ryohgo Narita's health conditions. Yen Press announced its English-language license for the SH novels in October 2020.