Admittedly, the books we tend to use are really quite dry, so I'm not sure if you'll be interested in these.
Although I will say that Kagan's book gets a bit more entertaining when you are sufficiently armed with page flags.
(If I recall, green flags marked interesting maps, blue flags marked important Athenian info, red flags marked important Spartan info, and yellow flags were because I ran out of flags :)
The main resource we used for the Philip campaign was simply titled "Philip of Macedon" by Nicholas Hammond. It is unfortunately out of print. Another one we used was "A History of Macedonia", by Hammond and Griffith. Volume II covers 550-336 BCE, which means it covers the Peloponnesian War and Philip's reign.
The main resource we're using for Caesar is a book simply titled "Caesar" by Adrian Goldsworthy. Some editions have the subtitle "Life of a Colossus". This has a huge amount of detail, but is unfortunately sparse on maps. We also have a copy of "Julius Caesar" by Philip Freeman, which is shorter and definitely a more interesting read. Goldsworthy goes into a lot of detail that wouldn't be particularly interesting unless you're, oh I don't know, making a game about him ;)
One recent book I picked up was Legions of Rome by Stephen Dando-Collins, which is a nice summary of the information we know about each individual legion. Unfortunately there simply aren't very many records offering specific information about the legions during the Gallic Wars, but it's fun to scan through, and I'll definitely get to use it more if we ever do something Roman that's set later in history.
Of course we also have some original sources, like Thucydides' Peloponnesian War and Caesar's Gallic Commentaries. These classics are all available wikisource, so you don't really need to buy them unless having them in book form is useful to you.
From the art side, our artists have a lot of the Osprey books, because these books go out of their way to provide a lot of visuals. These are definitely fun to skim through.