authidx was written entirely by
Nelson H. F. Beebein 1996 in support of production of the book Case Studies in Mathematical Modeling---Ecology, Physiology, and Cell Biology. However, it was written to be generally useful for indexing of arbitrary LaTeX documents, and there is nothing in it that is specific to that one book.
Other books that have been produced locally with the help of authidx include Variational Methods for Structural Optimization and Theory of Composites
The latter was particularly complex, in that it has bibliographies in each chapter, uses an extended Chicago-like bibliography style, and has a book design that is quite different from typical LaTeX book styles.