Witt's study of the Isis cult focuses primarily on her fame outside of Egypt, but his research is impeccable and always fascinating in its detail. Isis in Rome. Isis in Santorini. Isis in Gaul. Isis just about everywhere in the Mediterranean world. Witt does an excellent job when gathering information about her festivals, cult objects and practices, and her place in widespread popular piety as a precursor of the Vigin Mary figure. There's also info on the gods of Isis' Egyptian entourage--Osiris, Anubis, Horus, Nephthys--and their respective places in the cult outside Egypt. All in all, a marvelous and ample treatment of one of the ancient world's most influential and enduring religious traditions.
This book is nearly 50 years old, but still in a lot of bibliographies on the Isis cult. It is written by somebody with a background in Egyptology, Classics and Ancient Philosophy and thus this is about the underlying philosophy, rather than the archaeological remains and the problems of the shifting evidence. It seems to be also very much written from the perspective of a Western Christian and Freemason, which shows in some of the arguments, but which also leads some rather interesting links between the Isis-Cult and Early Christianity and especially Orthodox Marian Veneration. Certainly worthwhile reading, not just for Isis, but possibly also as a model what may have happened between other aspects of paganism and Christianity in Late Antiquity.