If you're inclined towards the traditional Solesmes method with its flowing 2s and 3s of rhythm, I suggest "The Technique of Gregorian Chironomy" by Joseph Robert Carroll or the "Gregorian Chant Practicum" by Justine Ward (and Theodore Marier).

...told me that 'chironomy is no longer current with Solesmes"...

Small wonder!
And chironomy's not all -

"The Technique of Gregorian Chironomy" by Joseph Robert Carroll or the "Gregorian Chant Practicum" by Justine Ward (and Theodore Marier).

"Singing in God's Ear" by Dom Nicholson OSB.

Are any of these available online as PDF scans?

Cardine's Direction of Gregorian Chant is useful, but even more useful is study of the adiastematic neumes themselves. They are, after all, chironomic signs, not music notation in the modern sense.

The Rhythm of Plainsong: According to the Solesmes School by Doms. Gajard (cited in Parish Book of Chant 2^nd ed. p. 310 on Plansong Rhythm)

In reply to this most recent question, there are no such books. The placement of the ictus within the classic-Solesmes system is more or less fixed according to the rules developed by Dom Mocquereau. It is often useful for a beginning conductor wishing to follow the method to mark all the ictus on the music. This can be extremely useful, so that you're not constantly trying to count backwards by twos. If you look at older harmonized collections according to the Mocquereau school, you can often glean the ictus placement, since they only change bass notes on downbeats.

The placement of the arsis and thesis for larger rhythmic structures is much more fluid. Mocquereau writes somewhere that one often has to adjust for the choir and the liturgical context, etc. Are the singers dragging and showing too little energy? Then give them more arsis gestures to fire them up. Are they rushing and going a little out of control? Then use more thesis gestures. Also the various classic-Solesmes authors don't always agree in their chironomy. You can compare Suñol, Ward, Mocquereau, Gajard, etc. and see that. Again, it's sort of an art form and dependent on context. It may be a good idea for aspiring conductors to draw the chironomy in, either using the swooping notation of Mocquereau or the handy abbreviations t. and a. used by Suñol.

Adiastematic notation refers to any notation that is not specific with regard to pitch. This would refer to most of the early manuscripts of chant. Fully diastematic notation would be notation on a staff. In the early centuries, there were also partially diastematic notations, that give some hint of pitch based on the relative heights of the signs, without having a staff that gives total clarity on pitch.