28 CFR §540.70

Except when precluded by statute (see § 540.72), the Bureau of Prisons permits an inmate to subscribe to or to receive publications without prior approval and has established procedures to determine if an incoming publication is detrimental to the security, discipline, or good order of the institution or if it might facilitate criminal activity. The term publication, as used in this subpart, means a book, booklet, pamphlet, or similar document, or a single issue of a magazine, periodical, newsletter, newspaper, plus such other materials addressed to a specific inmate such as advertising brochures, flyers, and catalogs.