A treatise explains a single subject. Some legal treatises are very comprehensive, extensive, and well-documented with references to primary and secondary sources. They are written by experts. While they are not binding authority, the courts sometimes cite to renowned treatises.
For lists of well-regarded treatises, look to the Treatises tab in our LibGuides on various subjects. Morris Cohen and Kenneth Olson have compiled a list of treatises on almost 50 subjects in Appendix B of their Legal Research in a Nutshell.