Hi Victor Leung, publishing a book it's hard. Maybe you have done an excellent job but unfortunately, as you mentioned, selling it is hard...!
I don't know how other people do it so I don't have any good advice for you... I'd say though that building an audience helps... and social media helps a tone with that... try to promote it in Twitter, LinkedIn, your company, etc...
p.s. I clicked on the link and it seems broken, you may want to check it :)