At last, at last, I'm pleased to announce that there is a source for understanding the old-style Solesmes approach to rhythm: The Rhythm of Plainsong by Dom Gajard (1943).
This marvelous book strikes me as the best overall defense of the Mocquereau approach to the rhythm of Gregorian chant, with Gajard clearly explaining, in non-technical terms, what is historical, what is deduced from musical understanding, and what is pure speculation.
He makes an amazingly persuasive case! Indeed, it strikes me that he settles the issue, and I'm just sad that it has been unavailable for many decades. In any case, it is back so at last we have a clear defense of the way the sacred music colloquium has been teaching chant now for many years. This is also the way most monasteries in the world sing it, save a few that have experimented around with alternative theories.
I don't need to say this, but it is clearly the answer to the view that chant has no inherent rhythm and must sung as if it were entirely text driven. I can imagine that one reading of this will set semiologists to rethink their position.
Print and read it if you can. It is nothing short of thrilling. And thank you Scott Turkington for pushing me to find this long-lost treasure!