Keegan, J. (2003). Intelligence in War: Knowledge of the Enemy from Napoleon to Al-Qaeda. London, UK, Pimlico.

Case stories of intelligence (from the Battle of Abukir, Shenandoah Valley, German-English sea battles in WWI, Crete, Midway, the U-boat war, and the hunt for the V-1 and V-2) and its strategic importance in warfare.

Main point (p.23):
"It is the intrinsic difficulty of communication, even, indeed above all, for the agent with ‘access’,  which limits his – or occasionally her – usefulness in real time. By contrast, the enemy’s own encrypted communications, if they can quickly be broken, will, of their nature, provide intelligence of high quality in real time.
The history of ‘how, what, where, when’ in military intelligence is therefore largely one of signal intelligence. Not exclusively, human intelligence has played its part and so, latterly, has photographic and surveillance intelligence. In principle, however, it is the unsuspected overhearings of the enemy’s own signals which have revealed his intentions and capabilities to his opponent and so allowed counter-measures to be taken in time."

On keeping the Napoleonic expedition to Egypt a secret: "Gossip helped to refine the picture. Some of the academics who were to accompany the expedition began to boast, a notorious failing of clever men leading unimportant lives."

From the conclusion (p398):
"[…] it strikes this author that the organization of intelligence-gathering and subversion within the same body is undesirable. Subversion is a weak way of fighting, differing from conventional warfare by the total unpredictability of its results; moreover, in a democracy, it is always liable to disavowal by legitimate authority and denunciation by authority’s political opponents. Intelligence-garhering, by contrast, can yield conflict-winning outcomes and , if securely and soberly conducted, is an activity only those of ill-will can condemn.
      Yet, in the last resort, intelligence warfare is a weak form of attack on the enemy, also. Knowledge, the conventional wisdom has, is power; but knowledge cannot destroy or deflect or damage or even defy an offensive initiative by an enemy unless the possession of knowledge is also allied to objective force."