Jeff Cheeger, James Simons, Differential characters and geometric invariants, pages 50–80 in: Geometry and Topology Lecture Notes in Mathematics, vol 1167. Springer (1985) (pdf, doi:10.1007/BFb0075212)
Jeff Cheeger, Gang Tian, Anti-Self-Duality of Curvature and Degeneration of Metrics with Special Holonomy, Communications in Mathematical Physics volume 255, pages391–417 (2005) (doi:10.1007/s00220-004-1279-0)