If you're into angelology, this is probably the best reference book there is. Nearly every named angel (including not only those from apocrypha, but texts including Christian, Jewish, Muslim, Mormon, and Hindu traditions) is included here. You can find all the different hierarchies of angels described here. It really is an encyclopedic reference. Writers who want to use angels from mythology would be well advised to refer to Davidson's work; many other dictionaries of angels are just condensed summaries of this book.