A good short treatment of analysis of covariance is given by Snedecor and Cochran (1980, Chapter 18). The book by Huitema (1980) is well-informed, thorough, and reasonably elementary. Among psychological texts, Maxwell and Delaney (1990) give an extensive discussion with a positive outlook on Ancova. Reichardt (1979) gives an illuminating discussion of Ancova with nonrandom groups, some of which is summarized in Section 15.5 on quasi-experimental design. I am indebted to Anthony Greenwald and Charles Reichardt for helpful comments on this chapter.