Some of the best publications in aviation come from our very own Federal Aviation Administration.  And,  they're all available on the web free!  Most are also available in printed form for your convenience, but the web is a great way to browse or do reference work.  Here are some of my favorites -- all highly recommended reading.

• Aeronautical Information Manual
• Airplane Flying Handbook
• Aircraft Weight and Balance Handbook
• Pilot's Handbook of Aeronautical Knowledge
• Aviation Weather
• Aviation Weather Services
• Student Pilot Guide
• Practical Test Standards - Private Pilot Airplane
• Instrument Flying Handbook
• Instrument Procedures Handbook
• Plane Sense