P.F. Chisholm
Patricia Finney
Sir Robert Carey, a nobleman in Elizabethan England:

A Famine of Horses (1994)

A Season of Knives (1995)

A Surfeit of Guns (1996)

A Plague of Angels (1998)

A Murder of Crows (2010)

An Air of Treason (2014)

A Chorus of Innocents (2015)

A Clash of Spheres (2017)

Written as Patricia Finney (real name)
Simon Ames and David Becket, secret agents for Queen Elizabeth, in 1580s England:

Firedrake’s Eye (1992)

Unicorn’s Blood (1998)

Gloriana’s Torch (2003)