One of my most useful reference books for this period and earlier is the WRG book by Nigel Stillman and Nigel Tallis, Armies of the Ancient Near East 3,000 BC to 539BC, which is unfortunately out of print. I read on the Society of Ancients forum recently that this will be back in print soon, perhaps with an updated bibliography like Duncan Head's classic Armies of the Macedonian and Punic Wars 359 BC to 146 BC. Good news for all the biblical fans out there!