Book excerpt: Two Girls, One on Each Knee: A History of Cryptic Crosswords is an audaciously constructed book on the pleasures and puzzles of cryptic crosswords and their linguistic wordplay, from Alan Connor, the Guardian's writer on crosswords On 21 December 2013, the crossword puzzle will be 100 years old. In the century since, it has evolved into the world's most popular intellectual pastime: a unique form of wordplay, the codes and conventions of which are open to anyone masochistic enough to get addicted. In Two Girls, One on Each Knee, Alan Connor celebrates the wit, ingenuity and frustration of setting and solving puzzles. From the beaches of D-Day to the imaginary worlds of three-dimensional puzzles, to the British school teachers and journalists who turned the form into the fiendish sport it is today, encompassing the most challenging clues, particular tricks, the world's greatest setters and famous solvers, PG Wodehouse and the torturers of the Spanish Inquisition, this is an ingenious book for lovers of this very particular form of wordplay. Note: The book begins with a puzzle in a standard 15-by-15 grid which incorporates all the basic clue types. The answers are also the chapter titles. Alan Connor writes twice-weekly about crosswords for the Guardian. He has contributed pieces about language for the BBC and the Guardian and works in radio and television, writing for Charlie Brooker, Caitlin Moran and Sue Perkins. His most recent writing was A Young Doctor's Notebook, a TV adaptation of Mikhail Bulgakov stories starring Daniel Radcliffe and Jon Hamm.