The magician gives a spectator a thick book -- perhaps a travel book with text, maps, and photos. The spectator is asked to choose any page at random and concentrate on it. The magician then describes the page exactly, including what is pictured on it and every word of text.
There are two methods for accomplishing this trick, and both of them use a real, unprepared book.
If the magician wishes to face the spectator, the book must be memorized beforehand. This can be done more easily than you might expect -- the trick is to think of the book not as a lot of words, but as a collection of pictures that happen to be of book pages. The magician then makes a mental association between each page and some familiar sequence (such as the pages of another book that has already been memorized), allowing any page to be instantly called to mind.
During the performance, the magician knows which page the spectator turned to by the most simple and direct means possible -- counting the pages.
If the magician is willing to turn away from the spectator (to "avoid peeking," of course), the trick is even easier. The magician just keeps a second copy of the book in a pocket and takes it out when the spectator is out of eyeshot. By asking something like, "So, for example, what page are you on?" the magician can instantly locate the page in his copy of the book and read it to see what should be "psychically" revealed.