How can anyone call him 'forgotten' ... You might call it an insult to the man.
Welchman's "Hut Six Story" is one of the best known of the earlier books on the subject. The bound volume that i have is from 1982. In my view he is one of the better known.
Hodges' great book on Turing came out in 1983.
Unknown to the general public would perhaps be a better description.