Legal treatises are written by legal scholars, and typically contain a detailed analysis of a single area of law.
Treatises are often referred to by their original author: Scott on Trusts or McCormick on Evidence.
LaFave on Search & Seizure is a multi-volume treatise devoted entirely to a single amendment of the U.S. Constitution. It is a comprehensive work that attempts to list and analyze every significant statute and court opinion on this topic. The full title is: Search and seizure: A Treatise on the Fourth Amendment. You can find this six-volume set in Westlaw Edge.
A Hornbook is a single-volume treatise, published by West, written specifically for law students. These are books that cover a single topic, but not in so much depth or detail. For example, there is a hornbook that covers all of constitutional law in one volume. Hornbooks generally correspond to law school courses, and are designed to supplement casebooks.
There is also a series of paperback books called Law in a Nutshell, also published by West. You may already own the Nutshell on Legal Research, as it is one of the recommended titles for this course. Nutshells are short and simple overviews of an area of law. They make no attempt to be comprehensive, but instead offer a brief introduction to the topic.
Hornbooks and Nutshells are both available through the West Academic Study Aids suite.
A legal treatise does not necessarily contain the word "treatise" in its catalog record, so you cannot rely on finding one in the library catalog using this keyword.
As you become a more experienced legal researcher, you will become familiar with the major treatises in your chosen area of law.
Lexis+ has a Treatises, Practice Guides & Jurisprudence collection
Westlaw Edge has a Texts & Treatises collection
Bloomberg Law has a Books & Treatises collection, linked from the homepage
Treatises will always have a table of contents, and they should also have an index.
For the most part treatises are complete in one volume at the time of printing. Always check to see if there is a more recent edition.
Multi-volume treatises, like LaFave, are updated with annual pocket parts, so check in the back of the book to see if there is one.