Pilot's Handbook of Aeronautical Knowledge FAA-H-8083-25A

"The Pilot’s Handbook of Aeronautical Knowledge provides basic knowledge that is essential for pilots. This handbook introduces pilots to the broad spectrum of knowledge that will be needed as they progress in their pilot training. Except for the Code of Federal Regulations pertinent to civil aviation, most of the knowledge areas applicable to pilot certification are presented. This handbook is useful to beginning pilots, as well as those pursuing more advanced pilot certificates." (Summary from the preface)
Study of the handbook should include the PDF from the FAA, which is available at http://www.faa.gov/regulations_policies/handbooks_manuals/.