Butler is the standard, classic source of biographical information on the principal saints. It is arranged by the saint's feast day. The other books describe how individual saints have been represented in works of art and are very helpful in understanding the scenes in which they are depicted.
Retelling the stories of figures from the Old and New Testaments, these books are illustrated with full-color reproductions of art works to illustrate how these stories have frequently been represented in the visual arts.
The first of these books discusses both secular and religious iconography by broad themes or activities (e.g., envy or expulsion). The others are classics in the field. Even if your French is not that good, it is worth consulting Reau or van Marle. All deal with the variations and evolution of major iconographic themes. Bibliographical references and numerous illustrations are also provided.