 Meyer Viol (1995a, 1995b) contain further proof- and model-theoretic studies of the epsilon calculus; specifically intuitionistic epsilon calculi.  Here, the epsilon theorems no longer hold, i.e., introduction of epsilon terms produces non-conservative extensions of intuitionistic logic. Other investigations of epsilon operators in intuitionistic logic can be found in Shirai (1971), Bell (1993a, 1993b) and DeVidi (1995). For epsilon-operators in many-valued logics, see Mostowski (1963), for modal epsilon calculus, Fitting (1975).