At most, secondary sources are persuasive authority.
Good for legal definitions. Start here if you are unfamiliar with a term.
Black's Law Dictionary
Ballentine's Law Dictionary
Good for a general overview. Like a general encyclopedia but specific to law. Multi-volume sets.
American Jurisprudence 2d (AmJur) and Corpus Juris Secondum (CJS) are the most widely used national encyclopedias.
Legal periodicals include law journals, law reviews, bar journals, commercial journals, and newspapers.
Subject specific discussion of the law. Depth varies. Some are also jurisdiction specific. The West Academic Digital Study Aids subscription includes the Concise Hornbook Series of one-volume treatises designed for law students.
Nimmer on Copyright
Dobbs' Hornbook on the Law of Torts
Maine Environmental Law Handbook by Bernstein, Shur, Sawyer & Nelson
Maine Family Law by Jon D. Levy
For more secondary material located in the library go to the secondary sources lawguides.