The Golden Legend - A very popular medieval book of saints' lives, written between 1260 and 1275 by Jacobus de Voragine.
Internet Medieval Sourcebook: Saints' Lives - Ancient, Byzantine, and medieval hagiographic original texts, in translation and otherwise, with basic data on the cult of saints, emphasis on historical understanding of the texts. [Note: the word "cult" is a technical term referring to the religious practices surrounding devotion to saints.]