A treatise is a scholarly commentary on the law usually written by a law professor or lawyer, often on the editorial staff of the publisher. It contains comments, citations to cases and other sources of the law, and internal finding aids like an index.
A hornbook is a textbook containing the basics of a given area of law. When you are looking for the fundamental concepts of a well settled area of law a hornbook can help.
The Nutshell series is a collection of concise and comprehensive guides to various areas of law as well as pratical guides to skills like trial and clinical skills for students and lawyers.
A Restatement is a form of legal commentary published by the American Law Institute (ALI). The ALI promulgates a rule-like source stating the common law of many legal topics such as contracts and torts.
Online guides are guides (similar to this one) from other law schools that may further help you find resources on this topic.