Constantine II is one of those Roman Emperors who no one gets excited about, despite having a relatively long reign and even possibly dying of natural causes. He's probably best known for being the baddy in the life story of Julian - a paranoid, crafty emperor who had a lot of his family wiped out and eventually tried to kill Julian, and who lost a heap of battles.
Crawford sets out to rehabilitate the man's reputation, while not being blind to his faults... It's exhaustively researched - as in a really detailed book. A lot is known about Constantinus, due in part to all the military campaigns he was involved in (many of them civil wars).
Occasionally the book seems to lose focus as Crawford goes off on other tangents - Julian Shapur, etc - but I understand why he did it. It's hard to imagine a more definitive account.
Crawford thinks Constatinus has gotten a bad wrap but isn't a massive fan of the man - he slaughtered too many family members, let too many incompetent killers loose in the kingdom (eg Paul). But he did do less long term military damage to the Empire than say Julian. He's a "solid" Emperor - competent, tired, no visionary but no disaster. A very strong book from the gang at Pen and Sword.