The selection of biographical works in English of Capetian rulers are actually fairly limited than generally assumed. My very elementary advice is: to search not with 'Philip II', but either with 'Philip Augustus' or 'Philip August' (and the spelling of 'Philip' itself is not always constant).
Anyway, the following should be included in almost every academic book list on him and most of them also appear in the wikipedia page in English about him:
- Baldwin, John W. The Government of Philip Augustus : Foundations of French Royal Power in the Middle Ages. Berkeley: U of California Pr., 1986: detailed academic analysis of his reign as well as his bailiffs. Perhaps a little too heavy for non-academic first-time readers on this topic.
- Bradbury, Jim. Philip Augustus : King of France, 1180-1223. London: Longman- Routledge, 1998: is the standard one, but not so detailed.
- Duby, Georges. The Legend of Bouvines: War, Religion, and Culture in the Middle Ages. Berkeley: U of California Pr., 1990: Originally written and published in French, and strictly speaking, not the biography solely about King Philip II. Nevertheless, I'd recommend this book since it offers the atmosphere of the time under the later part of Philip's reign, both for his Northern French follower aristocrats as well as his foes like King John of England.
The biographical work on his contemporary political rivals, Richard I, Elienor of Aquitaine, and his family might be also worth checking:
- Hanley, Catherine. Louis : the French Prince who Invaded England. New Haven: Yale UP, 2016.
- Turner, Ralph V. Eleanor of Aquitaine : Queen of France, Queen of England. New Haven: Yale UP, 2011: Turner wrote many books and articles on the Plantagenet like Richard and John, not only Eleanor.