Albert E. Hurd and Peter A. Loeb. An introduction to nonstandard real analysis, Pure and applied mathematics, no. 118. Academic Press, Orlando etc. 1985, xii + 232 pp. - K. D. Stroyan in collaboration with W. A. J. Luxemburg, Introduction to the theory of infinitesimals, Pure and applied mathematics, no. 72. Academic Press, New York, San Francisco, and London, 1976, xv + 326 pp.