In the design of clinical trials particularly in the regulatory environment of the FDA the determination of the appropriate number of subjects to recruit for a well-powered statistical hypothesis test ot tests is extremely important. This topic is often neglected in statistical texts, even the elementary ones. It has also been neglected in many of the statistical software products that are available. Recognizing this need Janet Elashoff and Ralph o'Brien were among the first to construct tools for sample size determination and calculation of statistical power. There are now many software products available including nQuery Advisor (Janet Elashoff's product), Power and Precision, PASS, and StatXact. For group sequential and adaptive design methods thare are products such as East, PEST, SeqTrial+ and ADDPLAN to mention a few. Ralph O'Brien's package has now been modified and implimented in SAS with the procedures PROC Power and PROC Glmpower.
This book by Jacob Cohen preceded these developments and became a classic reference for researchers particularly those in the social sciences including behavioral scientists as mentioned in the title. I know of only one other competing book from that time period that being "How Many Subjects" by Helen Kraemer of Stanford University.