A legal treatise is a commentary or discussion of a specific area of law published by a private publisher.   There are thousands of treatises.  Many are single volumes, many are multi-volume sets.  If a legal encyclopedia provides a little information about every area of law, a treatise provides a lot of information about a single area of law.   Features:

• commentary or discussion
• focuses on a specific area of law
• refers to, but does not contain, statutory and case law
• refers to additional forms of secondary and non-authority (i.e. encyclopedias & digests)
• considered secondary authority

Restatements of Law (above right) and Collier Bankruptcy Cases (above right) are a treatises.