that I adopted was his use of chalk pastels as part of his shading / weathering methodology.
I had used chalk pastels for years as a basic method for replicating dust, dirt and rust, but Mr. Greenland also applied (mostly dark colored) pastels using a fine tipped brush around raised details as a method of shading which was more subtile (and controlable) than highly contrasting washes. This was a new technique to me at the time and one that I adopted and still use sometimes. I think it was also a key to "The Tony Greenland Master Class Look," if that's where you're trying to go.
Although I never fully bought into his use of semi-satin finishes, that was the most revolutionary (evolutionary, more to the point) technique that I took from his work.
Too bad about losing your copy of his book. I believe that it's still available in re-print, though, from Osprey. I still leaf through my copy from time to time looking for inspiration.
Finishing techniques are an evolutionary process, and each new generation has built on the work of the trend-setters before it: Shep Pain, Verlindin, Tony Greenland, MIG, Zaloga and now Wilder and Rinaldi (here in North America), just to mention the guys who come to mind that have had the most influence on my finishing styles.
(And even MIG's "cutting edge" use of pigments was borrowed from Japanese model builders who I suppose will remain unknown here in the west - but were freely acknowledged by MIG in his early writings on the subject.)
If you're looking for the present cutting edge of finishing technique evolution, you might find some of the articles by Adam Wilder and Mike Rinaldi pointing you in the right direction, although Tony Greenland's work still gives much to aspire to.
(Their articles in MMiR are excellent. BTW: Pat Stansell - You could do worse than just gathering up all those finishing and weathering articles ASAP and publishing them in a single special edition. Thin, but packed chock full of good, useful how-to info. I'd buy a copy to keep near my work bench. How about spiral bound so it stays open easy?)
At any rate, don't forget that Greenland's finishing methods didn't just rely on Humbrol paints, per se, but were a combination of materials and application techniques that combined to give his work its unique look.