These are the bare minimum of things to read before starting to design Greek typefaces.
A primer for Greek type design (2002)
In Language, Culture, Type. John D Berry (Ed.), ATypI & Graphis
The pre-publication version of the text published here has some images replaced for copyright purposes, but otherwise is at a near-final draft state.
There are also some useful comments on parallel script development in the booklet produced by Microsoft to document the development of the ClearType typefaces, Now read this (2003). According to a recent Typophile thread, it may eventually be available as a PDF.