Because I recently bought some CDs with music of Ástor Piazzolla and John Tavener, I tried to find some background information.
The very first biography of Piazzolla (1921-1992) has been published by Oxford University Press in 2000. (update March 2011: strangely enough this seems not to be available anymore from the OUP websites).
There seems to be a URL www.piazolla.org but that is not working. (update March 2011: it should be www.piazzolla.org – note the double zz…)
A biography and further links on Tavener (born 1944) I did find here on the Schirmer website.