There are three editions of the British National Corpus, the original first edition (1994), the BNC World Edition (2001), and BNC XML edition (2007).
A good source of information is Guy Aston & Lou Burnard, 1998, The BNC Handbook, Edinburgh: Edinburgh University Press.
This guide does not attempt to represent all the facts about the BNC in detail since the BNC site itself and the Aston & Burnard book contain so much definitive information. Instead it concentrates on what you can do with the BNC using WordSmith Tools.
Obtaining the BNC
You can find it via this link.