Ninjas versus Slave Drivers! Book 2 in the Shadow Warrior saga.

When Ran, the Shadow Warrior, is dispatched to investigate rumors of an invasion massing to the north, he signs on as protection for a trade caravan. But once underway, the caravan is attacked by an overwhelming force of slavers who take Ran and several others captive. Dragged into the bowels of the earth as slave labor for a despot‑in‑exile eager to reclaim his throne, Ran must make a desperate escape with his friends if he has any hope of uncovering the truth about the invasion above ground. But there's more at play than a mere power struggle in the catacombs, and the earth does not give up its secrets easily—even for Ran, the Shadow Warrior.

Listen to the author discuss the book here on the Baen Free Radio Hour.