This isn't an exhaustive list, but it identifies some key starting points.
Gordon and Gavin's blog articles about their work:
Part 1: The background story
Part 2: Introduction, and why the cipher matters
Part 3: The codes and ciphers described in Codes and Ciphers
Part 4: The examples used in Codes and Ciphers
Part 5: Gavin finds an error in one of D'Agapeyeff's examples
Other online resources:
The Wikipedia article: This contains a machine-readable copy of the cipher.
Robert Matthews' article: This is the article that we used as a starting point for our work.
Published print resources:
D'Agapeyeff, A. (1939) Codes and Ciphers: A History of Cryptography. Meridian Books.
This is the book that contains the cipher. The cipher only appears in the first edition. It's usually fairly easy to find a copy of the first edition.
A photo of Gavin's copy of the first edition