Kline, P. (1999). Handbook of Psychological Testing(2nd ed.). London: Routledge. P.13
Despite the dangers of boosting the reliability of a test by making the items highly similar to each other, in which case validity is reduced, reliabilities should ideally be high, around .9, especially for ability tests. Certainly alphas should never drop below .7, a value stressed by both Guilford (1956) and Nunnally(1978). The rationale and proof of these claims are bound up in psychometric theory and are given in Chapter 3.
For 0.7; Nunnally,J . (1978). Psychometric Theory. New York, McGraw-Hill.
For 0.8; Nunally, J. C., & Bernstein, I. H. (1994). Psychometric theory (3rd ed.) New Yort: McGraw-Hill.
Abell, N., Springer, D. W., & Kamata, A. (2009). Developing and Validating Rapid Assessment Instruments. Oxford University Press.