Some sites written in Englsih:

The International Shakuhachi Society

The Gunnar Jinmei Linder Shakuhachi Site (Click on "This is Shakuhachi")

The Shakuhachi World of John Kaizan Neptune (Click on "Flute Making")

The Shakuhachi Mailing List Web Archive

Shakuhachi Home Page (Comprehensive, but written in Japanese)

~Masato