Original citation for testing for statistical equivalence using two one-sided P values.

The original reference is:  

 D.J. Schuirmann, A comparison of the Two One-Sided Tests Procedure and the Power Approach for assessing the equivalence of average bioavailability, J. Pharmacokinetics and pharmacodynamics, 115: 1567, 1987. 

The book by Wellek, cited below, explains the entire field of testing for equivalence.