Just don't take literally the reference about 5.3 million Persians invading Hellas, because then you'll start to have serious issues with Herodotus' sanity :)

Herodotus' Istoria is available online (in English, of course) at the Gutenberg project site and maybe the Perseus project as well (I am not sure) but I'd strongly suggest a hard copy of a good translation. It contains - despite it's problems... you see, it is the first history book - a wealth of data regarding all the cultures the Greeks have encountered and it also provides a vivid image of the world at that point of time.

If you want to know any particulars regarding the battle that are not in the book (or you can't find them) ask away. I have finished a month ago a huge (12 magazine pages) article on the Thermopylae battle so I have several things in my mind. Just ask specific questions.