This nifty little novel assumes that magic in Elizabethan times worked exactly like Elizabethans thought it did, that devils are real and spells efficacious (and some folks thought witches weren't necessarily in league with them).
Oh, and in this novel Sir Philip Sidney and Christopher Marlowe aren't dead (well, yet). Sir Philip survived his wounds in the Netherlands and was therefore alive to save Marlowe from that knife in the tavern (a foiled assassination attempt).
The authors, Melissa Scott and Lisa A. Barnett, have a pretty good understanding of Elizabethan history and culture, and it shows. You can smell danger just where you should (non-magical danger, that is; I have no experience with magic, so I couldn't say), courtiers are trying to get influence (or laid), Catholics are out of favor in a dangerous way and life is pretty miserable all around. In this uncertain time, Elizabeth's horoscopist predicts the beheading of her successor, touching off Sir Philip's trip to Scotland to see James safe from magical threats.
Complicated? You bet. Detailed? Yup. Fun for Elizabethan geeks? Absolutely.
Did I mention Marlowe is a spy?
The Armor of Light by Melissa Scott and Lisa A. Barnett is published by NESFA Press, who bring you people like Terry Pratchett.