Makoto,
Official documentation on NI-VISA and the VISA API can be found at National Instruments' website http://www.ni.com/manuals and http://www.ni.com/visa
I would also suggest the following manuals for reference:
http://www.ni.com/pdf/manuals/321074c.pdf
http://www.ni.com/pdf/manuals/321073b.pdf
http://www.ni.com/pdf/manuals/321074d.pdf