I was intrigued by the title of this book as I had seen Mithra mentioned on a QI repeat and knew nothing about him. When The Mysteries of Mithra popped up as the free Book of the Day on ForgottenBooks.org I tapped download immediately.
The book is quite short at 239 pages and spends a fair bit of its space describing what is not known - which isn't helpful. The initial chapter examining the history of the religions from which Mithraism came is dry with lots of dates and names I'd not read before. I nearly gave up but am glad I persevered as the remaining text covering the actual beliefs, spread through the Roman Empire, practices and art, is interesting.
I had the impression that Mithraism pre-dated Christianity, but the two seem to have begun concurrently and, despite their differences, have much in common - the 25th December nativity date being one such curious 'coincidence'. Also, apparently, our enthusiasm for astrology can be traced back to the followers of Mithra!
I am pleased to have read this book and have enjoyed Googling lots of new long words to discover their meanings. The phrasing and style is dated, which is excusable in a text of more than a century old, but once I got past that, it was a very educational few hours' study.