While browsing the Wikipedia I came across a reference to IL-2 Sturmovik http://en.wikipedia.org/wiki/IL-2_Sturmovik_%28game%29
On this page were links to pages with data for aircraft in the game.
For example the ME109 http://en.wikipedia.org/wiki/Messerschmitt_Bf_109
As part of our manual we could link to these pages to provide real world data for aircraft, while still writing our own game related material. The same could be done for game objects, such as artillery, ships etc.