My copy arrived few weeks ago
Grainger is making history of Seleukid kingdom, he is not focusing on military history of Seleukids. In my view he is not military historian (at least for Hellenistic period - I didn´t read his books about other periods). He is excellent for Hellenistic history, history of Hellenistic dynasties, Hellenistic politics, Hellenistic diplomacy. In his books he describes battles very shortly. His books are not like The Seleucid army and Judas Maccabaeus by Bar-Kochva - this are still the best books about Seleukid military history.
But it will be great to read some new specialized book about Seleukid military and campaigns with great illustrations