Shapur II was actually born already king, as he was the posthumous child of the previous king Hormizd II and was crowned while still in the womb. I can't help but wonder what would have happened if the child had been born a girl. There isn't actually all that much about him on the internet, mainly brief mentions in general histories of Sassanid Persia.
Wikipedia (usual caveats apply) has an article. Rozaneh magazine has two extracts from Kaveh Farrokh's Shadows In The Desert: Ancient Persia At War, the second of which covers Shapur II's reign and Julian the Apostate's invasion of the Sassanid empire. Otherwise, there is a portrait on a silver dish, now in the British Museum, and some coins.
Jona Lendering does have an article on Ctesiphon, including a link to a satellite photo of the remains there.
