XEL
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I'm not very familiar with the series, as after "The Price of Loyalty," I lost all desire to continue exploring it.
I remember reading a book about X-COM a long time ago, and I enjoyed it. According to reviews, "The Humble Genius of the Underdark" is very well written. Also, according to the same sources, the main characters in "Legacy of the Giants" are similar to the "Dark Others" from "The Price of Loyalty," led by Lyke, who spend every chapter drinking beer and boasting about their awesomeness.