De Rerum Natura + The Gospel of Thomas
Not a combination I would have ever expected, but once I realized the latter was building its ideas on top of the former's atomism and evolutionary thinking, the combo suddenly clicked and I was looking at philosophical ideas not only aligned with where I'd been at previously, but advancing my thinking significantly.
Probably some of the most intriguing ideas from antiquity I've ever seen, and much more advanced than I'd have envisioned I'd find.