This article is from the Legal Research FAQ, by Mark Eckenwiler with numerous contributions by others.
The law of contract, with its roots deep in the common law, has
inspired a number of exhaustive treatises, as well as sundry other
resources. The two most authoritative (and voluminous) works are the
multivolume sets _Corbin on Contracts_ and Williston's _Law of
Contracts_. The answer to nearly any question on contract law can be
found in either of these. If your library is more modest, a good
sourcebook in a pinch is the more readable, compact, and modern
_Farnsworth on Contracts_.
In addition, an important source of law are Articles 1 & 2 of
the Uniform Commercial Code, a model code of contract law that has
been adopted (sometimes with minor modifications) by most states.
Likewise influential is the American Law Institute's _Restatement
(Second) of Contracts_, a summary of commonly accepted principles of