Steel String and Classical are not different styles of playing guitar they are just different guitars in term of physical properties. Get any guitar you want, Acoustic or Electric and you can play any style on it, albeit some styles will sound better on Acoustic/Electric...
I disagree. I think I know of only one example of someone who played classical on a steel string, and that was probably Agustin Barrios. I also disagree the styles are not different. They are EXTREMELY different.
For nylon string, I would highly recommend the book Pumping Nylon. I learned some stuff about proper technique.
I studied for two years with the guy who wrote that. Its a good book, the most popular technique book ever for the style, but I didn't agree with 100% of it. However, even he understood the individual differences of any given person's physiology.
When I used to be an avid musician, I used a dilution of Carlevaro exercises, particularly the right hand stuff. Sort of something I came up with between the advice of a couple of other world-class musicians. I completely leave out l.h., and worked on the r.h. for warm-ups, as its the r.h. that is unbelievably difficult. The Carlevaro books are very pricey, and perhaps not easy to find.
Gosh, classical is so much more exigent than pop/rock. Anyways, for the first style, another (and very difficult) method to technically fool-proof a work is to apply severley dotted rhythms to any given work. Then use the reverse dotted rhythms. Very difficult, and will really tighten it up.
For contrapuntal music, and this is even more difficult, learn to play any given set of voices, and sing the remaining. VERY DIFFICULT. While it helps solo stuff, I think this is even more beneficial for chamber works.
-jostenmeat