The Deepest Poison is a precursor story for Beth Cato's Clockwork Dagger series. It is from Octavia Leander's time serving as a medician in the Caskentian army. The story is told from Miss Percival's perspective, and explains her enmity toward Leander.
The story is OK as explanation but it's kind of blocky as a story in itself. Read it just for information on the series, I'd say. 2 stars.