Written by an intelligence professional, this treatment of Civil War espionage reflects his background; he frequently interjects his opinions and provides deep detail for operational topics. Although Markle's informal and enthusiastic style is quite readable, the book's topical organization and exhaustive treatment of some fairly arcane topics make it more useful for researchers than general readers. In fact, the book's final five chapters have a quasi-reference organization; featuring Markle's discussions of all known Civil War spies, they would alone make the book worth considering for academic libraries. Public libraries may want to look elsewhere, but this is the most general account in print.