Roundworld is in trouble again, and this time it looks fatal. Having created it in the first place, the wizards of Unseen University feel vaguely responsible for its safety. They know the creatures who lived there escaped the impending Big Freeze by inventing the space elevator - they even intervened to rid the planet of a plague of elves, who attempted to divert humanity onto a different time track. But now it's all gone wrong - Victorian England has stagnated and the pace of progress would embarrass a limping snail.

In the 'fantasy' universe of the phenomenally bestselling Discworld series, everything runs on magic and common sense. The world is flat and million-to-one chances happen nine times out of ten. Our world seems different - it runs on rules, often rather strange ones. Science is our way of finding out what those rules are. The appeal of Discworld is that it mostly makes sense, in a way that particle physics does not.

The fourth book in the Science of Discworld series, and this time around dealing with the really big questions, Terry Pratchett’s brilliant new Discworld story Judgement Day is annotated with very big footnotes (the interleaving chapters) by mathematician Ian Stewart and biologist Jack Cohen, to bring you a mind-mangling combination of fiction, cutting-edge science and philosophy.Marjorie Daw is a librarian, and takes her job – and indeed the truth of words – very seriously. She doesn’t know it, but her world and ours – Roundworld – is in big trouble.

Authorised by Mr Lipwig of the Ankh-Morpork and Sto Plains Hygienic Railway himself, Mrs Georgina Bradshaw’s invaluable guide to the destinations and diversions of the railway deserves a place in the luggage of any traveller, or indeed armchair traveller, upon the Disc.

A weathercock has risen from the sea of Discworld and suddenly you can tell which way the wind is blowing. A new land has surfaced and so have old feuds. And as two armies march, Commander Vimes of Ankh-Morpork City Watch has got just a few hours to deal with a crime so big that there's no law against it. It's called 'war'.

A Shivering of Worlds. Deep in the Chalk, something is stirring. The owls and the foxes can sense it and Tiffany Aching feels it in her boots. An old enemy is gathering strength. This is a time of endings and beginnings, old friends and new, a blurring of edges and a shifting of power. Now Tiffany stands between the light and the dark, the good and the bad. As the fairy horde prepares for invasion, Tiffany must summon all the witches to stand with her. To protect the land. Her land.

As the mighty alien fleet from the very latest computer game thunders across the computer screen, Johnny prepares to blow them into the usual million pieces. And they send him a message: We surrender. They're not supposed to do that! They're supposed to die. And computer joysticks don't have 'Don't Fire' buttons. But it's only a game... isn't it?

Truth! Justice! Freedom! And a hard-boiled egg! Commander Sam Vimes of the Ankh-Morpork City Watch had it all. But now he's back in his own rough, tough past, without even the clothes he was standing up in when the lightning struck. Living in the past is hard. Dying in the past is incredibly easy. But he must survive; he has a job to do.

Finding himself alone on a desert island when everything and everyone he knows and loved has been washed away in a huge storm, Mau is the last surviving member of his nation. He's also completely alone - or so he thinks until he finds the ghost girl. She has no toes, wears strange lacy trousers like the grandfather bird and gives him a stick which can make fire. Daphne, sole survivor of the wreck of the Sweet Judy, almost immediately regrets trying to shoot the native boy.

It begun as a sudden strange fancy...Polly Perks had to become a boy in a hurry. Cutting off her hair and wearing trousers was easy. Learning to fart and belch in public and walk like an ape took more time. And now she's enlisted in the army and searching for her lost brother. But there's a war on. There's always a war on. And Polly and her fellow recruits are suddenly in the thick of it, without any training, and the enemy is hunting them.

Suddenly, condemned arch-swindler Moist von Lipwig found himself with a noose around his neck and dropping through a trapdoor into...a government job? By all rights, Moist should be meeting his maker rather than being offered a position as postmaster by Lord Vetinari, supreme ruler of Ankh-Morpork.

Sam Vines is a man on the run. Yesterday, he was a duke, a chief of police, and an ambassador. Now he has nothing but his native wit and the gloomy trousers of Uncle Vanya. And if he can't make it through the forest to civilisation, there's going to be a terrible war. But there are monsters on his trail. They're werewolves - and they're catching up. Sam Vimes is out of time and already out of breath....

Who's murdering harmless old men? Who's poisoning the Patrician? As autumn fogs hold Ankh-Morpork in their grip, the City Watch has to track down a murderer who can't be seen. Maybe the golems know something - but the solemn men of clay, who work all day and night and are never any trouble to anyone, have started to commit suicide....

To the consternation of the patrician, Lord Vetinari, a new invention has arrived in Ankh-Morpork - a great clanging monster of a machine that harnesses the power of all of the elements: earth, air, fire and water. This being Ankh-Morpork, it’s soon drawing astonished crowds, some of whom caught the zeitgeist early and arrive armed with notepads and very sensible rainwear.

It is a truth universally acknowledged that a policeman taking a holiday would barely have had time to open his suitcase before he finds his first corpse. And Commander Sam Vimes of the Ankh-Morpork City Watch is on holiday in the pleasant and innocent countryside, but not for him a mere body in the wardrobe. There are many, many bodies and an ancient crime more terrible than murder. He is out of his jurisdiction, out of his depth, out of bacon sandwiches and occasionally snookered and out of his mind, but never out of guile. They say that in the end all sins are forgiven.