Here is a pretty good
booklist divided by category. This should help her find whatever she's looking for. There is also a
list of books to avoid. Although a Wiccan site, these
book reviews should help too.
Remind your sister to read with a notebook and pencil handy, and that any book worth reading is
worth reading three times.
Here are some books that I highly recommend:
The Golden Bough by
Sir James G. Frazer is good for Paganism, magic, and religion in general. It's in the public domain.
Triumph of the Moon by Ronald Hutton is a great book on the modern history of Pagan witchcraft. Although it focuses on Wicca & witchcraft, it is a history book rather than a practical guide. Hutton is a professor of history at the University of Bristol and is a
leading authority on the history of paganism. It's a dry academic book, and may be a tough read for your sister.
Greek Religion by
Walter Burkert is a book that I consider mandatory if your sister wants to study ancient Greek religion.
Anything by
Joseph Campbell -- he dedicated his life to the study of myth. Here is a video excerpt from the interview
The Power of Myth.
Magick Without Tears was written by
Aleister Crowley at the end of his life. I consider it an excellent introduction to Ceremonial Magick in plain English. Not a necessary read if your sister isn't interested in CM. Nevertheless, she may find it beneficial to read Chapter I.
This reading list should get your sister started. Remind her, however, not to get too attached to one author - it is important for her to criticize even the books she agrees with.