Paul Stephenson's Constantine: Unconquered Emperor, Christian Victor (2009) is a solid narrative aimed at a more general audience, and will be far cheaper than any book aimed at academics. Another option would be Michael Kulikowski's The Triumph of Empire: The Roman World from Hadrian to Constantine (2016), which covers a much longer time period but provides an up-to-date narrative of the immediate context of Constantine's reign, since it's hard to get away from long-term trends when thinking about the changes that took place under the first Christian emperor. For a much more in-depth look at the sources, Timothy Barnes' Constantine: Dynasty, Religion and Power in the Later Roman Empire (2010) is a good overview, though some of the interpretations in his book are not representative of what other academics think. Another more academic choice would be the Cambridge Companion to the Age of Constantine (2011), which has various chapters on different aspects of this period written by experts in their respective fields.