"The 85 ways to tie a tie" is a book, written by two Cambridge University researchers: Thomas Fink and Yong Mao. It is a slightly tongue in cheek mathematical look at the theory behind tying knots, and how to apply the abstract ideas of knot theory to an everyday object: the necktie.
The book starts with a brief introduction to the development of ties. Apparently, the first embryonic forms of knotted neckcloths were found on statues in China dating from the reign of the first Emperor, Qin Shih-huang-di (259-221 BC)! The development and metamorphosis of the cravat (the predecessor of the tie) is followed through the ages, via Ancient Rome and the Balkans, through to Renaissance Europe. It is during this period, during the 16th and 17th century, that fashion, as we have come to understand it today, was born, and the cravat or tie was embraced as a distinctive, flamboyant and easily customised accessory.
Now for the bit you've all been waiting for. There are diagrams of exactly how to tie 85 unique tie knots, and a detailed history (a sort of tie etymology) of fifteen of those. The diagrams are surprisingly clear, and if those fail, Fink and Mao have come up with a foolproof coding system for each step of the knot. All the classics are there, from the Four-in-hand (that's the one you probably do) to the slightly more adventurous Windsor right through to the downright wacky Balthus.
It is astonishing to think that only four of these eighty-five knots are in widespread use: what a waste of topological effort!
After seemingly endless pages of weird and wonderful knots comes the maths, which isn't as daunting as one might expect - anyone with a good level of mathematical or scientific education should grasp the concepts without too much bother. Don't worry if even the thought of an equation brings you out in a rash though, the maths is a mere four page appendix, and not required to appreciate the book fully.
Overall, I think this a fantastic book to have on your bookshelf. It is a great talking point, and allows you to mock your friends' egregious necktie efforts with that little bit more conviction! However, if you find that "The 85 ways to tie a tie" is becoming less of a bookshelf fixture than an everyday dressing companion, you may have missed the point.
"The 85 ways to tie a tie", Thomas Fink & Yong Mao, Fourth Estate, ISBN: 1841155683
Many thanks to Oolong for a pointer to the knot theory node.