If you are looking for background, the above suggestions are great and I think Nic is right on with Appian although the autobiographies are good as well.
If you are (like I was) desperate to find something on uniforms and color schemes, etc. Good Luck! There doesn't seem to be much out there, and disappointing from Osprey that they have nothing.
The one book that I found helpful for getting my Pontic Army together (FoG army list), was Stéphane Thion's
Le Soldat Lagide de Ptolémée Ier Sôter à Cléopâtre published by LRT Editions.
Don't worry that it's in French if you don't speak French because the whole book is full-frontal, computer-generated illustrations of Hellenistic troops (albeit Ptolemaic). It has a range of tunic colors and even answers what goes on standards - from this source, it seems that it is the first two Greek letters from the city the unit is from.
So, for my Pontics, I followed the uniform colouring and used sea-greens and sea-blues heavily in my palate because for some reason they seem to be the right colors in my own imagination for Pontus. And unfortunately, or perhaps fortunately, there's no evidence to say otherwise.
Like I said, if you are just looking for background info, you can't go wrong with the above suggestions. If you have a lead pile and don't have the first clue on how to begin painting them (like me), then I cannot recommend Thion's book enough (and it's not expensive, about the size of an Osprey book).
If you are interested, this is how I ended up painting my Pontics:
https://leadadventureforum.com/index.php?topic=103308.msg1284302#msg1284302