I have been using csound for a long time, it is by far the most powerfull
and flexible sound synthesis system out there (and free). It is essentially a programming language that allows you to do synthesis.
This kind of text based interface has its strenghts becuase I believe
it really allows you to concentrate on the nuts and bolts of how a
sound is created and manipulated. I think it is a great environment for
learning low level synthesis techniques. I definitely recommend the csound
book already posted. You should also check out...
http://csounds.com/ there is a plethora of information there.
Be sure to check out the music section, there are many examples
compositions. My only complaint is csound is not the best envorionment for
performing music. Although Csound5 just came out, and it does support
VST which helps.