I am interested in the numbered notation system (a.k.a. jiǎnpǔ), and I would like to know if there was any book in order to learn about the details of it (fingering, for example). I already read the Wikipedia page but most of its information is not sourced.
I am not asking about personal opinion about which work is the best/clearest/etc. but simply about the existence of a reference work.
I don't know Chinese, but I can deal with most European languages.