that's like asking if the C manual written in 1969 by Kernaghan and Ritchie is till relevant - of course it is, even though we have moved on to C++, the underlying pinnings of the language remain unchanged and therefore still relevant
The computer music tutorial is a thorough treatment of all things to do with computer music, including synthesis methods, various effects typically used in music/audio processing and a lot of background dsp information that you can't find in similar books under several $100's
the link I give is to a copy that is delivered at that price, which is not true of the amazon link, however, there seems to be some discrepancy in price as amazon owns the other company now - predatory pricing perhaps?