The United States Army Marksmanship Unit's Pistol Training manual is available free as a pdf download from various websites. It is a comprehensive document which lays down the theory and practice of accurate target shooting with the pistol, most or all of which applies to shooting air-pistol and CO2 pistols. It is NOT a manual of combat or battle shooting, but a very lucid explanation of how to get the bullet/pellet through the dead centre of the target.

http://www.giwera.pl/pliki/pmtg.pdf (pdf document only)

http://www.bullseyepistol.com/amucover.htm (navigatable by click-buttons)

Have a read and see how you can improve!