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.

The acclaimed Science of Discworld centred on an original Pratchett story about the Wizards of Discworld. In it they accidentally witnessed the creation and evolution of our universe, a plot which was interleaved with a Cohen & Stewart non-fiction narrative about Big Science. In The Science of Discworld II our authors join forces again to see just what happens when the wizards meddle with history in a battle against the elves for the future of humanity on Earth. London is replaced by a dozy Neanderthal village.

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.

Eleven-year-old Tiffany Aching wants to be a real witch. But a real witch doesn't casually step out of her body, leaving it empty. Tiffany does, and there's something just waiting for a handy body to take over. Something ancient and horrible, which can't die.

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.

Tiffany Aching put one foot wrong, made one little mistake... and now the spirit of winter is in love with her. He gives her roses and icebergs, says it with avalanches and showers her with snowflakes - which is tough when you're 13, but also just a little bit...cool.

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.

Tiffany Aching, the young witch from The Wee Free Men, A Hat Full of Sky, and Wintersmith is back in a new adventure featuring Discworld characters both familiar to fans (such as Tiffany, the Wee Free Men, Granny Weatherwax and Nanny Ogg) and new (meet Wee Mad Arthur, the Nac Mac Feegle on the City Watch whose only previous appearance was a brief cameo in Feet of Clay, and city witch Mrs Proust - a fabulous Pratchett creation).

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.

Football has come to the ancient city of Ankh-Morpork - not the old fashioned, grubby pushing and shoving, but the new, fast football with pointy hats for goalposts and balls that go gloing when you drop them. And now, the wizards of Unseen University must win a football match, without using magic, so they're in the mood for trying everything else.

Most of us grow up having always known to touch wood or cross our fingers, and what happens when a princess kisses a frog or a boy pulls a sword from a stone, yet sadly, some of these things are now beginning to be forgotten. Legends, myths, and fairy tales: our world is made up of the stories we told ourselves about where we came from and how we got there. It is the same on Discworld, except that beings which on Earth are creatures of the imagination - like vampires, trolls, witches, and possibly, gods - are real, alive and in some cases kicking on the Disc.

Dodger is a tosher – a sewer scavenger living in the squalor of Dickensian London. Everyone who is nobody knows Dodger. Anyone who is anybody doesn’t.But when he rescues a young girl from a beating, suddenly everybody wants to know him.And Dodger’s tale of skulduggery, dark plans and even darker deeds begins.

Mightily Oats has not picked a good time to be a priest. He thought he'd come to the mountain kingdom of Lancre for a simple little religious ceremony. Now he's caught between vampires and witches, and he's not sure there is a right side.

William de Worde, the accidental editor of Discworld's first newspaper, just wants to get to the truth. Unfortunately, everyone else wants to get at William. And it's only the third edition.

It's an offer you can't refuse. Who would not wish to be the man in charge of Ankh-Morpork's Royal Mint and the bank next door? It's a job for life. But, as former con-man Moist von Lipwig is learning, the life is not necessarily for long. The Chief Cashier is almost certainly a vampire. There's something nameless in the cellar (and the cellar itself is pretty nameless), and it turns out that the Royal Mintruns at a loss.