I'm not sure where to get them individually in the US but I know where you can get the whole set of 10 books (each dedicated to an individual tree species). They can be purchased from David at
www.miniature-bonsai.com. He is updating his site and you will only get a message page. However, if you email him at the email listed, I'm sure he can give you details. I second Al's endorsement as this is a wonderful set. I have many shohin bonsai books from Japan - other than these, the photo books are the most inspirational.
John Romano
RI, USA - the 'shohin' state
PS. If you go to Gun's site, click on the Japanese version and you can explore some of the links - there are many more images of tree development than on the English version.