Here are three "official" collections of Federal Government infomation designed to assist with the transition to a new Presidential administration.
The best-known of these is the quadrennially-published Plum Book (officially United States Government Policy and Supporting Positions), which is a guide "used to identify presidentially appointed positions within the Federal Government" i.e., the jobs that are up for grabs.
The U.S. General Services Administration publishes an authorized transition directory, described by the creating legislation as a "compilation of Federal publications and materials with supplementary materials developed by the Administrator that provides information on the officers, organization, and statutory and administrative authorities, functions, duties, responsibilities, and mission of each department and agency." For example, it includes a reference to the Plum Book.
The U.S. General Accountancy Office, one of the Federal Government's nonpartisan "watchdog" agencies, prepared this guide to policy issues facing the new administration. Includes links to recent GAO reports.