If you could materialize one lost ancient source out of thin air which one would you choose?
I'm going to cheat (sorta) and say I want the entire corpus authored by Marsyas of Pella,* who wrote a history of Macedonia as well as a book on the education of Alexander (no doubt meant to evoke Xenophon's Education of Cyrus), and maybe other things.
I've named Marsyas for years because I want that info on the early kingdom! And while I'm sure it was biased (probably heavily in places), it would be far superior to anything we've got. (May be [one of] the source[s] used by Curtius when he talks about "Macedonian custom.") Antipatros also wrote on early Macedonia, but it may have been less complete than Marsyas, so I want Marsyas.
If I can't have that, then Kallisthenes's official history. Again, heavily biased, but it would have data we can't get elsewhere. Or Kleitarchos, to see how closely Diodoros and Curtius (and Justin-Trogus) used him, and maybe what Plutarch and Arrian used, as well.
Oh, yeah, and Aristotle's book of letters to Hephaistion. ;-)
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*Marsyas of Pella was the much younger half-brother of the Diadoch Antigonos Monophthalmos (One-Eye), via their father. That’s why I said his histories would almost certainly be biased in some respects.