A “learned treatise” is something (usually a book) that is supposed to comprehensively cover a subject, usually a legal subject but sometimes a scientific subject. It is the kind of book that an expert would rely upon. So usually you use your expert to testify about how important the treatise it, how what it says is authoritative, and how it helps your case. But sometimes you don’t have an expert. Now what do you do? Well, if the other side has an expert, you try to get the other side’s expert to validate your learned treatise and then get your facts in using their expert. This is risky, but it can work with enough preparation. Another way to do it (but it does not work very often) is to get the judge to take “judicial notice” of what the treatise says. Judges hate this if there is any controversy attached to the subject of the treatise. But sometimes it can work.