There are several journal articles and Edinburgh technical reports which describe the Extended ML formal development framework. Most have been written by Don Sannella and Andrzej Tarlecki . An introductory report on the specification and verification of Standard ML programs is Don Sannella's 1986 report, ``Formal specification of ML programs''. This may be followed by his 1989 report, ``Formal program development in Extended ML for the working programmer''.
Having read these reports, the reader will be well placed to appreciate two reports by Sannella and Tarlecki, ``Toward formal development of ML programs: foundations and methodology'' from 1989 and ``Extended ML: past, present and future'' from 1991.