A treatise focuses on a single area of law and is written by experts in that area. Treatises range from broad, multi-volume sets to narrowly focused one-volume titles.
Practitioners rely heavily on treatises for the detailed coverage and practical tips they contain. The content varies tremendously, but you can usually find thorough explanations of the law at issue along with citations to relevant cases, statutes, regulations, and other secondary sources. Some treatises also contain the full text of these materials in appendices.