Actually, he didn't publish that many books - some which were aimed at providing revival fodder and two, which in my opinion, were among the most important collections, 'Doomsday in the Afternoon' and 'Songs of the Travellers'
MacColl was not a scholar as such and he mistrusted academia - quite often his scholarship was somewhat dodgy and tied to his interested in promoting singing and encouraging others others to sing - the description of his speed-reading a book or article in preparation for a Critics Group meeting (can't remember if we used it in our programmes on him) was a revealing one.
The work he did on singing was, in my opinion, groundbreaking (and virtually impossible to discuss on forums like this due to the infighting that is one of our sad legacies of the early days of the revival).
I honestly don't know why he didn't publish his parents songs, or why academics like Goldstein, who was aware of them and referred to them often in song notes, didn't encourage him to do so or follow them up himself (he even produced an LP of Ewan and his mother).
I don't know if he remembered enough of them to make the exercise worthwhile - he remembered them being sung around the house and, as I said, many, possibly all of them were fragmentary and added to (Eddie Frow's description seemed to be a fairly accurate one).
MacColl wrote very little - articles mainly, even his autobiography was tossed off over a shortish period, then laid aside and eventually, published posthumously.
Face-to-face teaching, interviews and a very occasional radio programme, rather than the printed word were his thing (still reckon 'The Song Carriers has never been bettered in over half-a-century)
Sorry I can't be more help than that.
Jim Carroll