The three main books that have examined the record of the rifle musket and found it wanting (due to proper training of the troops that used them) are:

Paddy Griffith - Battle Tactics of the American Civil War
Brent Nosworthy - The Bloody Crucible Of Courage
Earl Hess - The Rifle Musket in Civil War Combat: Reality and Myths

All three are readily available.

Greg Biggs