If you are interested in learning about how to write user material subroutines for finite element programs then you might want to check out the the recent book Introduction to Computational Plasticity by F. Dunne and N. Petrinic (Oxford University Press, 2005). This book is one of the very few that actually discusses user material implementations in Fortran. It is particularly interesting to note that the book gives complete UMAT and VUMAT examples for ABAQUS, and that the exemplar code can be downloaded from the Oxford University Press website.
I have no connection with the authors and I have not examined or tested the subroutines that they present. It seems, however, that the book and the accompanying code examples could be useful for students and engineers interested in learning more about constitutive equations and UMAT development.