A starting point to see if our library has a treatise you need is to do a keyword search in SARA, the BLS Library online catalog, combining the subject matter you are interested in and the word "treatise." An example would be "torts" and "treatise" as a keyword search. If you are unable to find a treatise that satisfies your need in SARA, you can also do a WorldCat search to search the catalogs of other libraries. WorldCat is a database that contains millions of bibliographic records from thousands of other libraries throughout the world. Once you identify a treatise as being at a local library you can go to the library reference desk to obtain a pass to another library so that you can use the book there, or you can make an interlibrary loan request and we will try to borrow the book for you.
An excellent resource for identifying treatises currently in print is Legal Information Buyer's Guide & Reference Manual by Kendall F. Svengalis (2015). This comprehensive annual publication has an extensive annotated chapter on "Legal Treatises & Other Specialty Materials."
You may also want to look at the other LibGuides written by BSLS reference librarians on a variety of topics that link to important treatises.