There are very few theoreticians in tango but Mauricio Castro is one. I have been struggling through his books TANGO.Die Struktur des Tanzes I and volume II. Yes it would have been easier in English but those volumes were sold out at that time so I needed to activate my German, from early days in my life. (are available in English now)
It was a glad surprise to meet him at our local milonga at Pelé and chat for a while!