...Someday, I will write an alternate-history where Cicero sides with Caesar in 49, and things go somewhat differently.

I enjoyed the Tom Holland book about the Persian Wars but I have a sense that people who actually know things about Persia don't think much of it; I suspect that it suffers, like his other work, from a kind of superficial and uncritical use of sources. I was going to suggest Pierre Briant's book (From Cyrus to Alexander -- I haven't read it but he's someone whose name I know, at least) but I just googled and it's ridiculously expensive to purchase.