Wikipedia – Has a fair bit of information.

Primary sources compiled by Prof. Derek Abbott – Derek Abbott is deeply invested in the case and has compiled a ton of primary resources as well as additional research into the case. Delving deep you can find the attempts at breaking the code, which he has set for his students as a task in the past and they come up with some interesting observations.

Amazon – The Unknown Man – Good and well written book on the case. I’m not sure if there are others, but this title was consistently recommended when I begun researching the Somerton man.