For a discussion of the Sokhotski-Plemelj formula see, for example, Joseph A. Cima, Alec L. Matheson & William T. Ross (2006). The Cauchy Transform. American Mathematical Society. p. 56. ISBN0-8218-3871-7. or Rainer Kress (1999). Linear integral equations (2nd ed.). Springer. p. Theorem 7.6, p. 101. ISBN0-387-98700-2.