Each of the files types are basically binary encoded. The ASCII portions are derived from the string references stored in the file formats data field. The only format that could truly be considered pure ASCII would be the decompiled models so that they can be read into 3ds max or gmax. The Key file is also relatively an ASCII file as well.
If your interested in making tools I would recomend visiting Torlack's Site (
http://www.torlack.com/) there you can download the C++ sourcecode to his various NWN tools. These can form a very good foundation for you to further understand the programming of the file formats.