as far as hardcopy goes
for introductions to twistors
selfAdjoint says he has Huggett and Tod
which is one of halfdozen introductions mentioned in
the Witten paper,
as introductions the paper recommended numbers
15, 28-32 from its list of references:
[15] M. A. H. MacCallum and R. Penrose, “Twistor Theory: An Approach To The Quantization Of Fields And Space-Time,” Phys. Rept. 6C (1972) 241.
[28] R. Penrose and W. Rindler, Spinors And Space-Time: Volume 1, Two-Spinor Calculus and Relativistic Fields, Volume 2, Spinor And Twistor Methods In Spacetime Geometry (Cambridge University Press, 1986).
[29] R. S. Ward and R. O’Neil Jr. Wells, Twistor Geometry And Field Theory (Cambridge University Press, 1991).
[30] L. P. Hughston, Twistors and Particles, Lecture Notes in Physics 97 (Springer-Verlag, Berlin, 1989).
[31] T. N. Bailey and R. J. Baston, eds., Twistors In Mathematics And Physics, London Mathematical Society Lecture Notes Series 156 (1990). 93
[32] S. A. Huggett and K. P. Tod, An Introduction To Twistor Theory, London Mathematical Society Student Texts 4.
I haven't ever laid eyes on any of these. But the fact that selfAdjoint already owns [32] Huggett and Tod is something of a recommendation.
I am still curious to know if anyone has found some sort of introductory twistor textbook ONLINE. Could one be buried in PF's "napster" links?