Most of the EB units are reconstructed from original evidence, that is from archaeological sources published in scholarly works. If you want some good books on the Hellenistic militaries dealing with arms and military equipment, look at the two volumes written by Nick Sekunda for Montvert Publications, titled "Seleucid and Ptolemaic Reformed Armies 168-145 BC." Volume 1 is on the Seleucid army and volume 2 is on the Ptolemaic army. Hellenistic Bactria is featured in "The Armies of Bactria 700 BC - 450 AD" by Valerii P Nikonorov, while some Osprey titles also deal with the Antigonid Macedonian army (just search their site). For a more scholarly work, you can also consult Sekunda's "Hellenistic Infantry Reform in the 160's BC."