Radio Navigation For Pilots by Bramson & Birch is in its third edition this compressive manual combines the use of an explanatory text with Aerad charts, and a realistic and thoughtful representation of an instrument panel.
The Instrument Flying Handbook is the primary FAA source for the Knowledge Exams that cover the IFR system as well as aerodynamics, flight instrumentation, attitude flying and navigation as they apply to the instrument pilot.
Instrument Procedures Handbook explains to instrument-rated pilots how best to use the system they've been trained for. The FAA's Instrument Procedures Handbook is a great resource for instrument-rated pilots!
The Practical Test Standards is a guide for students, instructors, and FAA-designated examiners to know what is expected of pilots during Instrument Rating (Airplane, Helicopter & Powered Lift) checkride.