For starters, the Nova Organi Harmonia is pretty great. Are you looking more for a technical guide as opposed to sheet music?

Geremia,

I've never seen the point I'm about to make in print, but it must be there somewhere:

How you accompany chant depends on two fundamental questions, which are "Does the organ constitute an independent voice?" and "Must the melody's notes (all or some of them) be in the right hand)?

If you are looking for technical instructions on accompanying Gregorian chant on the organ ad-lib, I would suggest anything by Henri Potiron who followed the Solesmes school of chant interpretation. His two chant accompaniment books were translated into English, and available online:

https://www.ccwatershed.org/2013/03/19/1933-potiron-gregorian-accomp-treatise/

https://www.ccwatershed.org/2013/03/19/1949-practical-instruction-plainsong-accomp/

Potiron was a very good Gregorian scholar, musician, and composer. His work on Gregorian modality is one of the best, but, unfortunately, not well known in the English speaking world. He also wrote full accompaniments for the Graduale Romanum and others (eg Kyriale, Vesperale, etc.) to help singers in average parishes and monasteries, which, at least in the French speaking world, became classics.

Geremia, had a look at both the books you recommended and they seem useful to me insofar as an organist might:
1. Want to imagine the exact opposite of their method and use that, or
2. Use their method as the basis for some kind of Durufle-esque choral arrangement (adding in a few tablespoons of impressionism for good measure).

To their credit, both writers have been clear about their principles... and I rather wish more authors would set out their work so neatly!

The NOH and its offshoots are excellent for accompanying chant, but far from ideal for leading chant from the organ (where some change on every ictus is really necessary to keep a less-skilled ensemble moving along)

It should, and that's why I said it is an excellent accompanimental resource. There is a limit to how much motion can be shown from the console when the singers are not a) fully in sync with each other as to the intended tempo of the chant, and b) wanting to drag in the first place. Adding notes to the accompaniment is the only way to accomplish this purely from the console, short of absolute witchcraft or beating the singers into submission elsewhere. If you have a conductor or a competent schola, this is a moot point.

@francis: Yes, NOH seems awesome.
Here it is as one giant 1.1GB bookmarked PDF.

@francis The link to the 1.1GB PDF works now.

There's also a torrent link or magnet link, which might be faster to download.