George, T. N. 1986. Biospecies, Chronospecies and Morphospecies, in The Species Concept in Paleontology, P. C. Sylvester-Bradley. pp. 123-128; pp. 134-137.

This covers biological and morphological species. You will by now have considered the species from a number of standpoints. There are strong reasons for supporting the species as a natural, biologically constrained unit, but at the same time, there are problems with it.