History of the Unicorn Run is a book about the legends surrounding the titular river. This book is only needed once to complete the subquest: A Bard's Request in Baldur's Gate. This book is sold by the Friendly Arm Inn. Another copy can be found at the top-floor of Ramazith's Tower. He will go hostile anyway after you freed the nymph Abela his rival kept prisoner.
History of the Unicorn Run: Bards and sages pass down the tale that the headwaters of the Unicorn Run are, in thruth, the Font of Life, and a cradle of fecundity. Each natural race is said to have emerged from the womb of Chauntea onto Toril at the river's source, and then traveled down the the Unicorn Run to the outside world. Some say that a daughter of Chauntea resides at the river's source to usher the newborns into the world, while others claim that Shialla midwifes the process.
Regardless of the truth, the elves, korreds, and halflings all agree that the Unicorn Run is sacred to life and a site of incredible purity. As a result, all three races have strong taboos about extented trips up the run, for if the river is ever fouled, then no new races will ever be born on Toril again.