The Cross Before Constantine: The Early Life of a Christian Symbol, by Bruce W. Longenecker.
In a sense, one could say this is a work of art history. The book makes a rather moderate claim in exhaustive detail, that the cross was used as a Christian symbol well before Constantine elevated it to the premier symbol in Christian graphic representations.
The cross was not the primary sign in this early period, nor did it have the prominence it did after Constantine, nor was it overly prominent in architecture. Nevertheless, early use of the cross as a religious symbol paved the way for its rise to prominence by Constantine and after.
Longenecker makes these claims with substantial and fascinating evidence. It's worth paging through this book simply to review the illustrations.
The most interesting insight, however, is the evidence that shows the cross having an apotropaic function. It was used as a symbol that sent a message to superhuman entities "that to mess with people associated with the cross is to mess with a supreme power--a power that even the forces of death cannot conquer" (187).
In other words, the cross did not have a liturgical or architectural significance... it was instead a sign of personal identity, and functioned as a ward.
Constantine's innovation was to harvest this apotropaic function for political purposes.