The Copyright Office provides short, succinct topic-based Circulars which provide great information if you are searching for a specific topic. Circulars cover the basics concepts of copyright law, provide highlights of policies and procedures of the Copyright Office, and registration issues for specific categories of works.

The Compendium is a robust resource produced by the Copyright Office which has much more in-depth information than the Circulars. The Compendium runs 1100+ pages but is fully indexed and can be searched.

When a copyright application has been denied, there is a Review Board to which an applicant can appeal the denial. After an appeal, the Review Board issues written determinations which have recently been archived online. You can select certain categories of information to limit the scope as well. These letters give a great break down of copyright law and reference pertinent cases. 

The Copyright Clearance Center has a useful learning portal for a review of the basics. 

This website is sponsored by the Stanford University Libraries, Justia, NOLO, LibraryLaw.com and Onecle and has a robust collection of information and articles on copyright. The site also provides case summaries, case docket information, recent copyright legislation proposals and a blog.

The Copyright Alliance is primarily a lobbying group that represents the interests of makers and creators. But, more than that, it offers an enormous trove of educational materials and resources for learning about copyright. The Copyright Alliance also posts free, topical. educational video content.  

Copyright and Creativity offers amazing free curriculum for teaching (and learning!) about copyright for people of all ages.