The only solo biography I know of is the one by Ernest Edward Bryant which is rather old. I know that there might not exist a work only focusing on Antoninus Pius so I hope you can at least recommend any quality work that will expand my knowledge on the emperor.
How's your German reading ability? Or French? Unfortunately, there are no modern English treatments of the topic that I can readily find. Just in case you do read German, here are the citations. And I tossed in one further more general survey of the period (in English!).
Michels, C. (2018). Antoninus Pius und die Rollenbilder des römischen Princeps: Herrscherliches Handeln und seine Repräsentation in der Hohen Kaiserzeit.
Rémy, B. (2005). Antonin le Pieux: 138-161 : le siècle d'or de Rome. Paris, France: Fayard.
Aumann, G. (2019). Antoninus Pius: Der vergessene Kaiser.
Grant, M. (1994). The Antonines: The Roman empire in transition. London: Routledge.