For captivating true life signals intelligence there are several books one can go to, including those by James Bamford on the American system (Puzzle Palace, Body of Secrets) but for really getting into the enormity of the challenges and the thrill of the individual code-breakers when they succeeded, this is the book I recommend.
It completely ignores the enormous contributions made by the Poles (who gave the English two Enigma machines at the beginning of the war) as well as the heroic deeds of Tommy Brown (youngest George Medal winner at 16, survived with code materials taken from a sinking German ship), but I have found no better novel to communicate the absolute goose-bump emotional roller-coaster that the Bletchley Park gang experienced.
If anything, this novel convey a human side to code-breaking that offsets the modern-day obsession with massive computers.
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