It may comprise multiple volumes or it may be only a single volume. A single-volume treatise is often referred to as a hornbook. In fact, you may already be familiar with hornbooks because they are commonly used by law students as study aids, especially in first-year courses. Some treatises cover a broad topic and some cover only a very narrow topic. Like other types of secondary sources, treatises can provide both background information and citations to relevant authority, but their coverage is both narrower and deeper than that of an encyclopedia.
Some treatises are merely expository, that is they do nothing more than state what the law is.
Other treatises provide an in-depth analysis of the law and include the author's personal insights. Such a treatise may examine questions like whether the law makes sense, whether the law is just, or whether the law is outdated and ought to be changed. In the hierarchy of secondary authority, treatises rank very high - particularly if they are interpretive or critical. They can be cited freely in memos and brief and can be very effective in bolstering a legal argument. Most of the treatises in our library are found in the stacks on the 9th floor.