WITH AN INTRODUCTION BY HOWARD MARKSOnce upon a time, opium (the main ingredient of heroin) was easily available over the chemist's counter.The secret of happiness, about which philosophers have disputed for so many ages, could be bought for a penny, and carried in the waistcoat pocket: portable ecstasies could be corked up in a pint bottle.Paradise? So thought Thomas de Quincey, but he soon discovered that 'nobody will laugh long who deals much with opium'.