"Plum Book" is the popular name for a publication officially titled United States Government Policy and Supporting
Positions. The Plum Book is "published by the Senate Committee on Governmental Affairs and the House Committee on Government Reform alternately after each Presidential election ... [it] lists over 7,000 Federal civil service leadership and support positions in the legislative and executive branches of the Federal Government that may be subject to noncompetitive appointment, nationwide. Data covers positions such as agency heads and their immediate subordinates, policy executives and advisors, and aides who report to these officials."
Electronic versions of the Plum Books (1996-Present) are posted on FDsys as one of the Additional Government Publications (Scroll down to "United States Government Policy and Supporting Positions (Plum Book)." You can buy print copies of the current edition from the U.S. Government Bookstore.
Older editions of the Plum Book are held by Federal Depository Libraries. You can locate these Libraries through a link on the GPO Access Plum Book page, above.