Another great free read from Karen Chance The Queen's Witch picks up the story straight after the ending of The Gauntlet. These stories are set in the same world as the Cassie Palmer / Dorina Basarab books but are much earlier in time and don't contain spoilers for either series.
I'd recommend reading both short stories in order though because this one would contain spoilers for The Gauntlet. They are both fast paced and fun reads though and fill in a bit of background information about the Silver Circle and Kit Marlowe. They're great for when you're looking for something quick to read.