Well when it comes to ancient sources Polybius is always a classic, Plutarch's Lives also looks at some of the famous folks from Hellenistic history but it is much later and many of the accounts Plutarch does not take from earlier authors are hard to verify so he is worth a read but I would not take everything on face value unless reading it in conjunction with other texts (preferably some modern ones as well).

For modern sources on the Wars of the Diadochi and the Hellenistic Period you can not go wrong with Peter Green's Alexander to Actium which looks at all of the Hellenistic kingdoms from Alexander's death to the death of Cleopatra. From Samarkhand to Sardis: A New Approach to the Seleucid Empire by Susan Sherwin-White and Amelie Kuhrt is really the only easily accessible comprehensive volume on the Seleucid Empire and you can read it in its entirety online.

For the Ptolemaic Kingdom I strongly recommend Egypt in the Age of Cleopatra by Michel Chauveau which is one the most comprehensive and well researched compendiums on Ptolemaic Egypt to date, although it tends to focus on the last century of Ptolemaic rule. All of those are quite pleasant reads and reasonably comprehensive, if you want to read further Chauveau, Green and Sherwin-White/Kuhrt provide rather extensive bibliographies to skim.