See the online study aids for Contracts
Treatises - Scholarly treatment of an area of law; often many volumes and usually updated annually. These titles were originally authored by big names in the field. We even refer to them by the name of the author. These titles are cited by courts.
Corbin on Contracts / Joseph M. Perillo
Murray on Contracts / John E. Murray
Williston on Contracts (A Treatise on the Law of Contracts) / Richard A. Lord
Restatements - A set of treatises on legal subjects that inform judges and lawyers about general principles of common law. They are published by the American Law Institute.