Gabriel Valiente has released a dictionary of Tango which contains discographies of most tango orchestras. It was meant to be released as an ebook, but is only available in paper edition at the moment. I think it looks interesting, but find that kind of information is generally vastly more useful when available electronically, in a format that is searchable and sortable etc. I hope Gabriel finds a way to release the full dataset as well as his book.
EDIT: There is also a kindle edition available now.