The International Committee of the Red Cross has made its 2005 study called Customary International Humanitarian Law freely available via its website. In otherwords an electronic version of the weighty volumes available in the LawBod at Internat 750 I62c with the advantage of the promise of “regular updates” – which will be in green text.
References to this database should be cited as ICRC, Customary IHL Database, followed by the URL of the document and the date last accessed.
There is also an online introduction by Jean-Marie Henckaerts.