Yep, Georg Heyde & Co. was located in Dresden, and operated up to the end of World War II. It's been said that the company survived the fire-bombing of the city, but folded after the establishment of the DDR.
You mention O'Brien's book, but if you're not aware, he wrote 4 editions of "Collecting Toy Soldiers". The second edition expanded on the first, but covered makers from around the world. Then he split American-made and all others into two books, edition three for American-made and edition four for foreign-made (foreign from our perspective). I'm pretty sure that there is a chapter on Heyde in Eds. 1 and 2 as well as in 4, though.
Heyde produced figures in a range of sizes, with the smaller sizes solid-cast in a soft alloy with a good percentage of lead. That made production easier, as the basic figure could be bent to shape, and accessories soldered onto ti. In the larger sizes, they produced hollowcast figures. The figures were relatively simple in detail, taken individually, but Heyde was famous for the display sets, and it's in these large displays that the simple detail of the figures is forgotten, when we look at the sheer size and scope of the displays. One set was the Triumph of Germanicus, depicting the triumphal parade that hero was granted after subduing the Germanic tribes. Another was the Sack of Troy. Other, smaller sets included various scenes of camp life. It can be said that before Britain establish itself, with the hollowcast technique, Heyde was the premier toy soldier maker in the world.
Prost!
Brad