Handbuch der Altertumswissenschaft, Bd.4, Römische Religionsgeschichte

Description via 3rd party site: This book, together with Dumezil's "La religion romaine archaique" and Wissowa's "Religion und Kultus der Römer" , is essential for understanding Roman religion habits.
This is not some mythological atlas or some kind of similar popular encyclopaedia that, covered with colourful images, tell you a bit of everything (but nothing in complete). This is serious work on Italic/Roman religion before it was taken over by and mixed with Greek.

Although specialists may disagree in details, since much in Roman religion is still uncertain, Latte's book can give us an adequate conception of Roman religion and its history. It contains almost anything that one may want to know about it; it is not likely to let one down. Following its fourteen chapters are four "Anhaenge" concerned with offering and prayer, Roman priesthoods, the Argei, and a list of Roman temples with dates of foundation. Following the index are six folded sheets presenting the twelve months of the Roman calendar; nine columns for each month present different sorts of information about each day. This is in every respect a necessary book for the student of Roman religion.