Jacob Cohen's A Power Primer in Psychological Bulletin (1992, Vol. 112. No. 1, 155-159) is a great place to start. A more extensive resource, also by Jacob Cohen, is Statistical power analysis for the behavioral sciences (2nd ed., Hillsdale, N.J. : L. Erlbaum Associates, 1988). The Cohen book has excellent introductory chapters if you want to better understand the purposes and terms associated with power analyses.