Besides being subject to Nondisclosure Agreements, Comey falls under federal laws governing the disclosure of classified and nonclassified information.  Assuming that the memos were not classified (though it seems odd that it would not be classified even on the confidential level), there is 18 U.S.C. § 641 which makes it a crime to steal, sell, or convey “any record, voucher, money, or thing of value of the United States or of any department or agency thereof.”

There are also ethical and departmental rules against the use of material to damage a former represented person or individual or firm related to prior representation. The FBI website states:

    Dissemination of FBI information is made strictly in accordance with provisions of the Privacy Act; Title 5, United States Code, Section 552a; FBI policy and procedures regarding discretionary release of information in accordance with the Privacy Act; and other applicable federal orders and directives.”