Terrorism and Counterintelligence Read this book. This book is a seriously foundational work for understanding how counterintelligence is applied in the real world.
Counterintelligence: Theory and Practice A textbook for professionals learning CI theory and how to apply it to protecting an organisation (typically an Agency). It is a bit dry at times, but very solid. The Kindle version is completely broken, but can be fixed easily in Calibre.
From Pablo to Osama This book is an excellent follow up to Terrorism and Counterintelligence as it also explores practical real world CI.