Anthony Horowitz has written several short stories, all considered canon in the Alex Rider universe. These stories have been printed in British newspapers and have appeared on the internet, the first being published in 2003. As well as these, Horowitz has also written additional chapters for his books, such as "Resistance to Interrogation" (Stormbreaker) and "Coda" (Snakehead). The extra Stormbreaker chapter can be found here while the extra Snakehead chapter can be downloaded from here.
Many of the short stories and extra chapters can be found on Anthony Horowitz's website
Alex Rider: Secret Weapon
(Published February 9, 2003 in The Funday Times)
Alex Rider is back at school, in a hurry for his class. He runs into a new teacher at the school, and she tells him her name is Miss Treat (Miss Summers in the American version) to which Alex humorously replies “You certainly are”. Miss Treat then reminds Alex not to run in the corridors.
A little while later, Alex is on a school excursion to a weapon museum when he encounters Skoda, a drug dealer whom Alex once put in jail after using a crane to drop the canal boat he'd been living in from a height into the middle of a police conference. His face, damaged from the accident, had changed since Alex last saw him.
Skoda pretends to be a security guard, though when Alex realizes who he is (he recognized him from the absence of his missing front teeth) Skoda traps him on the ground, before attempting to stab Alex with a sword from the museum. Miss Treat then arrives and shoots Skoda with a tranquilizer gun before he can hurt Alex, and explains that she works for MI6, who sent her to his school to keep an eye on him. After Alex thanks her for saving him, she once again reminds him not to run.
Alex Rider: Christmas at Gunpoint
(Published January 1, 2007 in The Daily Mail)
Part One - A thirteen-year-old Alex and his uncle Ian Rider are on a skiing holiday at Gunpoint, Colorado. Alex notices some strange men at the resort, as well as seeing his uncle behaving somewhat unusually around them. Alex is put in a difficult situation when he witnesses the kidnapping of Sahara Sands, a girl he met at the resort. He follows them.
Part Two - Alex has followed the kidnappers and finds out that Sahara has been kidnapped for the ransom of her father's laptop. When her father hands it over, Alex races down and snatches the laptop from the kidnappers, leading them on a high-speed ski chase to steal it back from him. When they are about to kill him, Ian shows up and saves Alex.
This short story is set five or six months before the beginning of Stormbreaker, meaning that this is the first short story chronologically. It was published in two parts, part two being printed a week after the first part. Notably, this story is written in the first person, from Alex Rider's perspective, and it is currently the only part of the series written this way.
Alex Rider: Incident in Nice
(Published November 9, 2009 in The Times)
Whilst on a short break with his friend from school, James Hale, in the South of France, James' enthusiastic uncle Andrew comes up with the idea that Alex should try Parasailing on his last day there. Alex and James' aunt, Celestine set out and Alex gets ready to parasail. However, whilst in the air, the driver of the boat suddenly collapses. Alex begins to worry as they continue heading out to sea, though a sudden wave causes the speedboat - and Kristof the driver - to shift, the steering wheel Kristof had collapsed on suddenly turning, causing the boat to do a U-turn and start heading back towards the beach. In order to avoid a horrific catastrophe, Alex fights his way down to the boat and manages to turn the boat away from the beach with less than seconds to spare, just missing the people in the water. As people gather around the speedboat and begin calling an ambulance for Kristof, Celestine walks out of the nearby ice cream parlor, and Alex approaches her, remarking that "It was certainly quite a ride."
This short story is set between the events of Point Blanc and Skeleton Key.
Alex Rider: Underground
(Published August 8, 2008 in the News of the World Summer Reading Special)
A civil servant working for the Ministry of Defence named Davenport is being followed by a man as he walks with a briefcase into Leicester Square tube station, on his way to North London. Alex walks by these men on his way to the cinema to meet some friends and watch a film, though recognizes the man following Davenport as the man he'd nicknamed "Spectacles", after he'd been mistakenly kidnapped by him and three others from St. Dominic's Hospital. Alex turns and begins to follow them, only to see Spectacles kill Davenport and take his briefcase. Noticing Alex, he immediately reaches for his gun, only for Alex to use karate to knock him onto the tracks as the tube train races out of the tunnel. Spectacles luckily lands in the crawl space between the rails, and is neither electrocuted or hit by the train. Trapped, the police and security services drag him out when the train leaves. It is said that it is publicly announced that Davenport had died of a sudden heart attack. Alex neither knows what the briefcase contained, nor why Spectacles was paid to steal it, speculating that some secrets were to be kept secret at all cost, and he is allowed to slip away. He missed the film.
This short story could possibly be considered AU (alternate universe) as Spectacles was killed in Ark Angel but this story appears to take place after the events of the novel, as the narrator comments near the beginning that Alex's kidnapping at the hands of the four men led him to "Ark Angel, the extraordinary space station orbiting the earth."
Alex Rider: A Taste of Death
(Published 2012 for World Read Day)
For UK users only and for app users only available in March on World Read Day 2012 or something. Alex Rider has recently returned from his second mission as a teenage spy for MI6, and life is back to normal. So normal, in fact, that the only reason for missing school is an appointment with the dentist.
But a chance encounter with a familiar face triggers Alex's suspicions and it isn't long until he's back in action fighting a terrorist conspiracy. Criminals make people sick with free chocolate samples. Chelsea is in an epidemic and everyone panics. They break out a fraud businessman from jail intending to torture him into revealing the location of all his hidden cash. Alex saves him and then has to go to the dentist.
Supposedly from Horowitz on Twitter:
"A TASTE OF DEATH is about a bad banker, a gang and a major poison outbreak..."