Board Thread:Questions and Answers/@comment-1620907-20151127200839/@comment-3055683-20170810215636

It does mention a brief tidbit about Guilliman reworking the Codex Astartes and the introduction of the newer Primaris squad types, but other than that, not much to go on. Although it is mentioned briefly in the 'Dark Imperium' novel in Chapter Nine, pg. 105 that by the end of the Indomitus Crusade sometime in M42, he had concluded his revisions to the Codex Astartes and had begun work upon a new book entitled Codex Imperialis, which would set down the principles of good governance long denied Mankind. Unlike his prior work, the Codex Astartes, the Codex Imperialis was intended to deal primarily with civilian matters.

The Codex Imperialis was to be a guide to the reorganisation of the Imperium of Man's political and socioeconomic institutions so as to restore the galaxy-spanning human realm to a state more in line with the Emperor of Mankind's original ideals of human betterment during the Great Crusade. It also was a work of supreme academic scholarship, intended to lay out the first truly accurate history of the Imperium from the time of the Great Crusade to the beginning of M42, unencumbered by propaganda and religious superstition in line with the original ideals of the Imperial Truth long-forgotten by the present-day Imperium.