The Bureau of Justice Statistics(part of the Department of Justice) publishes tables of felony convictions by type of conviction. Here is one such report from 2002, which shows that _about 5% of incarcerated felons in prison that year received a trial._ (Source: BJS, State Court Sentencing of Convicted Felons)
 explains more about the process. They also cite some academic literature. I didn't review the academic article cited, but BJA claims that their results show a similar result (that 90-95% of inmates pled guilty) (Source: BJA, Plea and Charge Bargaining).