Manuals

[AG-51_Glider]

These are the free official manuals by the FAA:

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-15B.pdf
Instrument Flying Manual

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/
Helicopter Manual

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/FAA-H-8083-25A.pdf
Handbook of Aeronautical Knowledge

http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/
Airplane Manual

And here is my all time Favorite
http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

Great additions to your library even if you wont get around to reading them all.