In the past, the question banks were limited and one could literally memorize the answers to the hundreds of questions possible. That has been changing for several tests, most notable the CFI part. Most pay third party companies that have compiled the questions and answers into different formats. Most of the tests you can still do this but the harder tests have been getting a lot of questions added.
As far as actual source material, the following is an example from Private Pilot. All of this can be found in electronic form online for free.
The knowledge tests for Private and Recreational Pilot exams are based on the following references.
Note:
14 CFR part 1 Definitions and Abbreviations
14 CFR part 39 Airworthiness Directives
14 CFR part 43 Maintenance, Preventive Maintenance, Rebuilding, and Alteration
14 CFR part 61 Certification: Pilots, Flight Instructors, and Ground Instructors
14 CFR part 71 Designation of Class A, B, C, D, and E Airspace Areas; Air Traffic Service Routes
14 CFR part 91 General Operating and Flight Rules
49 CFR part 830 Notification and Reporting of Aircraft Accidents
AC 00-45 - Aviation Weather Services
AC 00-6 - Aviation Weather
AC 20-43 - Aircraft Fuel Control
AC 60-22 - Aeronautical Decision Making
AC 90-48 - Pilots` Role in Collision Avoidance
AC 91-13 - Cold Weather Operation of Aircraft
Aeronautical Information Manual
Chart Supplements U.S. (formerly Airport/Facility Directory)
FAA-H-8083-1 - Aircraft Weight and Balance Handbook
FAA-H-8083-11 - Balloon Flying Handbook
FAA-H-8083-13 - Glider Flying Handbook
FAA-H-8083-2 - Risk Management Handbook
FAA-H-8083-21 - Helicopter Flying Handbook
FAA-H-8083-21 - Rotorcraft Flying Handbook
FAA-H-8083-25 - Pilot`s Handbook of Aeronautical Knowledge
FAA-H-8083-29 - Powered Parachute Flying Handbook
FAA-H-8083-3A - Airplane Flying Handbook
FAA-H-8083-5 - Weight-Shift Control Aircraft Flying Handbook
FAA-H-8261-1 - Instrument Procedures Handbook
FAA-P-8740-39 - Balloon Safety Tips
Navy Lighter-Than-Air Airship Flight Manual
Sectional Aeronautical Chart
www.faa.gov - Search for ACs
The latest revision of these references should be used.