I've just finished reading Thucydides, Xenophon, Donald Kagan's series about the Peloponnesian War, and several others, but now I'm having trouble finding anything covering the period between that and Alexander. Anyone have any recommendations? Looking for both ancient sources and modern histories. Thanks!
That period (403-336 BCE) was relatively peaceful aside from Phillip II. During this time you have the great minds of Plato, Socrates, and Aristotle making the "historical headlines" rather than wars and leaders. Obviously, I would recommend reading any of their works as many of them are easy to get through. The Internet Classics Archives have all of Plato's works translated in English.
If you are looking for war, then look no further than Phillip II's conquests. Phillip II of Macedon built the foundation of Alexander's army that marched triumphantly across the world. Phillip II used cavalry, albeit not as effectively, and infantry in great success. There are even those historians that believe Alexander would have been nothing without his father's influence and empire building prior to Alexander taking the throne. Here is a list of sources I recommend on the rise of the Macedonian Empire:
Phillip II: Greater than Alexander by Richard A Gabriel
"Book 16" of Bibliotheca Historia by Diodorus Siculus
Philip II of Macedonia: A Life of From the Ancient Sources by Bradford (may have a hard time finding this one)
Philip of Macedon by NGL Hammond (can probably find an e-source; Hammond may have been the foremost Alexander/Macedonia scholar in the world)
Minor Markle has a few good essays about Macedonian tactics and weaponry.
Demosthenes's speeches are also useful, especially the ones that involve Macedonia. Speech against Philip is available online.