A 100 year climbs out of his retirement home bedroom window in part to avoid his party and things just seem to happen to him. Forrest Gump (with a lot more intelligence) meets Douglas Adams, the main character does not have to do anything to find excitement it just seems to find him.
This book has a great twist in that the character you think is going to be this harmless very old man who is going to need to be protection is anything but. The way the book switches from the past to the present really works well and reveals the main character Allan Karlsson has been behind the scenes of many of the great events of the twentieth century (without really trying - Forrest Gump bit), as well as being probably one of the brightest people ever (speaks several languages, nuclear physics is not a problem, international diplomacy, etc) not bad for someone with only three years of formal education - this guy makes James Bond look incompetent. The switching between the past and present also does help develop the story.
The 'supporting' cast are also great in the present day story, including another character who is a polymath just because in the past he was getting money to be a student (but he did have to complete); but starts the story as a Hot-Dog seller. Who falls in love with a red-haired woman who is hiding an elephant on a Swedish farm. A crime boss whose lieutenants are in fact wiped out the others and has links I didn't expect. A quite lonely police officer. An ambitious prosecutor, who is looking for fame. A businessman who knows how to produce the best 'Swedish' chickens and 'Spanish' Watermelons.
This story is funny, good read that doesn't insult the reader, whilst still having a great feel-good factor.