The typical cypher type used back then was either the one-time pad or the Enigma Machine (lot of those about just post WW2 and only the British could decode them).

Book codes are typically used by smaller rings, as it is important to make sure everyone has a copy of the exact text and edition.

The suicide pill is, or was, a feature of WW2 espionage. Cold War spies were not encouraged to commit suicide when caught, as they could always be traded back to their homeland for captured opposition operatives.

If I had to guess, I'd say that these people were former Soviet operatives, possibly NKVD, who had defected. They kept in touch via the book code. I'd suggest that the defaced copy found in the car belonged to the dead man. Possibly he thought he'd been identified, and wanted to warn Thomson. This he did by tearing the key words out of the book and sending the book itself to her, possibly with a coded message inside. He then committed suicide rather than lead whoever was hunting him to his former lover and their child. Thomson, on receiving the book a few days later, realised what it meant and took steps to dispose of both his and her own copy. At which point Boxalls' copy simply becomes a book. This makes her reaction upon seeing the cast more natural, though it is possible that she thought the dead man had simply 'gone dark' rather than died.

This is my storytellers' mind at work, of course! This kind of thing calls for a le Carre or a Len Deighton to do it justice.