"God has endued the will of man with that natural liberty, that is neither forced, nor, by any absolute necessity of nature, determined good, or evil." - WCF 9.1.
In the four-fold state of man (before the fall, after the fall, after regeneration, in glory) man has the liberty of will to make voluntary choices. That means his choices are not forced or coerced to do something he does not want to do. But it is important to take note that there is a distinction between liberty and ability. Many stumble here. The will of man has liberty - We are never forced to do something we do not want to do in any of the four states. In that sense our wills are always at liberty. But that is quite different than the ability of the will. The ability of the will to be good or bad is not something that is retained in the fall. We will always, apart from the grace of God, choose what is bad since we are in bondage to sin. We still have the freedom to choose (without coercion) but choose evil by necessity due to a corruption of nature, and are unable to do otherwise.
Resources
Man's Natural Liberty by Robert Shaw
Does Man Have Free Will by John Calvin
The Will of Man by Derek Thomas
Since God Is Sovereign, How Are Humans Free? by R. C. Sproul
The Meaning of Man’s Will by R. C. Sproul
Freedom, Liberty and the Westminster Confession by Paul Helm
Free Will vs. Free Agency by J. I. Packer
Free Agency by John Murray
Free Will and Free Agency by John Reisinger
Grace Does Not Destroy Free Agency by R. L. Dabney
Do Human Beings Have a Free Will? by John Hendryx
21 Misunderstandings of Reformed Thinking by Sam Storms
How did the First Human Sin Happen? Various Reformed Authors
Free Choice in Reformed Theology by Seeking Understanding
Multimedia
Of Free Will (Ch.9, Sec.1) by Greg Bahnsen
Of Free Will (Ch.9, Sec.1) by Greg Bahnsen
eBooks
Human Nature in its Fourfold State (eBook) by Thomas Boston
The Bondage of the Will (eBook) by Martin Luther
A Treatise on Grace and Free Will (eBook) by Augustine
The Doctrine of the Will (eBook) by William Cunningham
Augustine & The Pelagian Controversy (eBook) by B B Warfield
Books
Bondage and Liberation of the Will by John Calvin
Willing to Believe by R.C. Sproul
What about Free Will?: Reconciling Our Choices with God's Sovereignty by Scott Christensen
The Bondage of the Will, Martin Luther