Here are some of Joel Greenberg's favourite books relating to signals intelligence.

Seizing the Enigma - David Kahn
Kahn, the leading writer on signals intelligence and code-breaking, gives a colourful account of the breaking of the German navy's Enigma system and its impact on the Battle of the Atlantic in WW2.

The Hut Six Story - Gordon Welchman
Welchman's book was the first to describe in detail how the British were able to read German wireless communications on an industrial scale during WW2. It is the seminal book in the story of Enigma and Ultra intelligence.

The Secret in Building 26 - Jim DeBrosse and Colin Burke
The little known story of the development of the American Bombe machine and the engineer who built it - Joe Desch.

The Enigma War - Jozef Garlinski
The heroic story of the work of the Polish codebreakers in the 1930s.

Most Secret War - RV Jones
The story of the role of scientific intelligence during WW2.

The Story of Magic - Frank Rowlett
An extraordinary account of how. the American Army's Signals Intelligence Service broke Japan's Diplomatic Cipher machines during WW2.

Colossus - edited by Jack Copeland
The first full account of the breaking of Germany's Lorenz cipher system and the building of arguably the world's first electronic computer.

Alan Turing, The Enigma - Andrew Hodges
The most complete account of the life and work of Alan Turing who was fundamental to the success of Bletchley Park during WW2.

Enigma, The Battle for the Code - Hugh Sebag-Montefiore
A well researched account of the Enigma story including detail about how French intelligence used information from a German traitor to aid the Polish effort against  the Enigma system.