I have only read 3 books on intelligence gathering and analytics, I found them all to be helpful. They were "Structured Analytic Techniques For Intelligence Analysis" and "Psychology of Intelligence Analysis" both by Richard Heuer Jr. and "Anticipating Surprise: Analysis for Strategic Warning" by C. Grabo. And while not really on the same topic, one of the best books I have ever read was "The Gift of Fear" by Gavin De Becker. The book is more about threat assessment and analysis than intelligence gathering, but in a way they do share some commonalities.