Lentwick's Arcane Chronicle
This old textbook on arcane matters is not actually magical itself. Studying the book for 1d4 rounds provides a +2 bonus on a Knowledge (Arcana) check, including when used with spells like Detect Magic. While the chronicle is not obviously illegal, it is old enough that some of the arcana contained within may be illegal if it were published more recently.
Historical Details
History
Copperday, Tailormoon, 1313
Hugo Dargent and his companions, Connie Furr and Lindie Asklepa, found a miraculously undecomposed copy in the body of a Shambler that had formed from the marinating books in the former-office of Swackhammer Fartrodden in the Understacks of Elderton University Library.Punishment: Depends what it contains, likely to get away with fine although could result in exile or banishment.