The Book of Heroes is the book left in the Sanctuary "For Jasper" by the Old King/Queen of Albion in Fable III. It details everything about Heroes and the purposes and rooms of the Sanctuary. Jasper is surprised that the Old King/Queen left it for him and uses it in order to give information to the Hero of Brightwall about the purposes of the Sanctuary.
It is a large, brown, hardback book, with the Guild Seal emblem on the cover. It was written by the Hero of Bowerstone, at the time of his/her use of the Sanctuary. It covers information about Heroes and the functions of the Sanctuary.