Specialists consider the Nova Organi Harmonia to be the best chant accompaniment volumes ever produced, but they are exceedingly difficult to find. Jeffrey Ostrowski with Corpus Christi Watershed has put all the volumes online. I forget how many thousands of pages, but it is an incredible achievement. Dazzling in every way. Congratulations to him. This is amazing work.
Controversies always break out on the subject of chant accompaniment of course. For many reasons, I prefer unaccompanied but, no, accompanied chant is not heresy, and these look to be very beautiful.