Four great books on this subject worth checking out, in no particular order, are The Greco-Persian War by Peter Green, The Achaemenid Persian Army by Duncan Head, Shadows in the Desert: Ancient Persia at War by Dr Kaveh Farrokh and, of course, Herodotus' Histories (the Landmark edition is really nice even if you already have the Penguin paperback).
Here are a few photos of the infantry:
|Greenwood and Ball, Hinchliffe and Old Glory - Immortals|
|Vendel - Egyptian Marines|
|Vendel and Old Glory - Persian Sparabara|
|Minifig and Newline - Persian Slingers|