There are a few, but Innocent III is so significant and a bit overwhelming as a person and a pope that it’s hard to cover everything in a single book. It seems like there are more books about his relationship with specific countries or his involvement in specific issues than there are about Innocent III himself!

But to begin with, here are a couple of book-length biographies:

- Jane E. Sayers, Innocent III: Leader of Europe (Longman, 1994)

- John C. Moore, Pope Innocent III: To Root Up and Plant (Leiden, 2003)

Also some collections of articles, which have some biographical chapters about Innocent:

- James M. Powell, ed., Innocent III: Vicar of Christ or Lord of the World? (Catholic University of America Press, Washington, D.C., 1994)

- John C. Moore, ed., Pope Innocent III and his World (Ashgate, 1999)

- Brenda Bolton, Innocent III: Studies on Papal Authority and Pastoral Care (Ashgate, 1995)

Then lots of books about particular aspects of his reign, such as:

- C.R. Cheney, Pope Innocent III and England (Hiersemann, 1976)

- Damian J. Smith, Innocent III and the Crown of Aragon: the Limits of Papal Authority (Ashgate, 2004)

And some edited primary sources:

- James M. Powell, trans., The Deeds of Pope Innocent III (Catholic University of America Press, 2004)

- Andrea Sommerlechner et al., ed., Die Register Innocenz III. - this is an ongoing series, started in 1964 and up to volume 14 now I think. It’s all in Latin with notes in German so I’m not really sure how useful that would be here...it’s just an example of the vastness of the sources for his life and reign. He had his nose in literally everything!

And if you're interested in the general history of the papacy in Innocent’s time, see:

- Colin Morris, The Papal Monarchy: The Western Church from 1050 to 1250 (Oxford University Press, 1989)