I think maybe Gertrude the Great was speaking of reading the docuмents referred to, not Fr. MM's books.
No, I meant the books. Have a look at Hostage to the Devil or Windswept House. They're both heterodox in morals, and Hostage to the Devil is at least erroneous in faith. Martin actually says that the priest is the one most at risk in an exorcism. What puerile, absurd, unorthodox, CRAP. He also carries on in an unorthodox manner about the contest of wills which he insists takes place between the devil and the exorcist. That's complete codswallop. He naturalises everything, ironically by pretending to talk about the supernatural. He was as much a naturalist, probably more so, as JP2.
Martin is on record confirming what old friends of his are on record asserting, which is that he lost the faith in the 'sixites, specifically in relation to the Person and natures of Our Lord Jesus Christ, as well as the Resurrection. He said that his fellow Jesuits started avoiding him because of his open heterodoxy. That might be true, but I'd say it was as much to do with his adultery as anything else. The man was a walking scandal, a wreck of a priest, and a wreck of a man.
He described in an interview later that his faith came back when he hit rock-bottom, driving a cab in New York. Yet he never made any public retraction of his public errors, nor did he ever make any kind of retraction of his assistance of the enemies of the Church at Vatican II.
In my judgement Martin was certainly guilty of adultery with Kaiser's wife, and he had other girlfriends as well. He was also demonstratively in the pay of non-Catholics seeking to influence the Council.
Of none of this did he repent in a way that anybody can verify. That should raise alarm bells with anybody who knows anything. But even without that consideration, it remains a public fact that he lost the faith and there is no equally public abjuration of error to undo his heresy. Therefore he stands as an unrepentant heretic.
And his books, as far as I have looked through them, are riddled with errors. They are also obviously works of fantasy. I don't know how much he actually had an agenda of misleading trads by talking about satanic rituals, with all of the technicolor and juicy detail, rather than the errors and heresies of V2 and the new mass. Maybe he was just making money by tailoring some paperback trash for a hungry audience which would be uncritical of anything which reinforced their idea that the Vatican had been taken over by evil, as it had.
Somebody once descibed him as a Walter Mitty. That seems to me very accurate. Consecrated a bishop in secret by Pius XII, allowed to read the Third Secret, running about performing dramatic exorcisms, in the know about everything, etc. It would be hilarious if only so many good people were not so blind about him!