Start with high-confidence ancient philosophy books, then narrow by subgenre or adjacent topics.
Lives of the Eminent Philosophers
Diogenes Laertius, Pamela Mensch
The Dangerous Life and Ideas of Diogenes the Cynic
Jean-Manuel Roubineau, Malcolm DeBevoise - translator
The Oxford History of Classical Reception in English Literature
Patrick Cheney, Philip Hardie
The Rise of Rome
Livy, T. J. Luce
History of Western Philosophy
Bertrand Russell
Humanly Possible
Sarah Bakewell
On the Heavens
Aristotle
The Politics
Kybalion: a Study of the Hermetic Philosophy of Ancient Egypt and Greece
Three Initiates