From the Compact OED:
noun a hired ruffian or unscrupulous subordinate.
 — ORIGIN from Greek Murmidones, a warlike people of Thessaly who accompanied Achilles to Troy.

From The American Heritage Dictionary:
NOUN:
1. Greek Mythology A member of a warlike Thessalian people who were ruled by Achilles and followed him on the expedition against Troy. 2. myrmidon A faithful follower who carries out orders without question.

I’m thinking that the book I read used the word as the AHD did in its second sense.