From the GPO:
The publication lists more than 9,000 Federal civil service leadership and support positions in the executive and legislative branches that may be subject to noncompetitive appointment.
First introduced in 2012, The Plum Book app allows users to view the publication in an easy to use mobile format and search the publication by agency, position title, location, appointment type, pay plan and level, tenure and term expiration, and vacancy.
The Plum Book is published by the House Oversight and Government Reform Committee and the Senate Committee on Homeland Security and Governmental Affairs alternately after each Presidential election.
The Plum Book is used to identify Presidential appointed and other positions within the Federal Government.
The list originated in 1952 during the Eisenhower administration. For 22 years prior, the Democrats controlled the Federal Government. When President Eisenhower took office, the Republican Party requested a list of government positions that President Eisenhower could fill. The next edition of The Plum Book appeared in 1960 and has since been published every four years, just after the Presidential election.