Fiction and non, featuring Mithras and Mithraism.

The Mithraic mysteries (Mithraism) was a Roman mystery religion or cult centered on the god Mithras, the Persian (Iranian) god of the sun (fitting well with the whole Sol Invictus motto). It was popular amongst the Roman Army in the 1st to 4th centuries AD/CE, and arguably influenced early Christian beliefs.

If you want to visit a Mithraeum (Mithraic Temple), the St Clement Basilica (Basilica di San Clemente) in Rome, right next door to the Colosseum, was built upon a 12th century church, which was built on top of a 4th century church (which doubled as a Mithraeum, complete with a marble altar stone on a pivot, for alternately worshipping Christ or Mithras), which in turn was built over a 1st century Mithraeum, complete with a central ritual chamber and statue. There's still water running in the ancient pipes down there, and you can walk through each area - they were never razed, rather 'incorporated' into successive churches). A nifty virtual tour is available on the Basilica di San Clemente website

There is also a Mithraeum in London that can be visited (Walbrook). Search "London Mithraeum" for more info. There are MANY in Germany, and there is a joint European museum project currently underway - visit mithra-project.eu for more info.

This list is a catch-all, from cheap paperback thrillers with rather dubious historical content or a near-total lack of research behind them, to academic dissertations and archaeological treatises.