1) some introductory material on receptor theory:
http://en.wikipedia.org/wiki/Receptor_theory
Yes, I know Wikipedia, a more comprehensive intro to basic neuro is "Principles of Neural Science, 5th Ed" (http://www.mhprofessional.com/product.php?isbn=0071390111)... It's large book but the de facto standard.
2) a reference to some basic pharmacology with respect to opioid receptors that includes a summary discussion on receptor interactions (i.e. full agonists/antagonists/partial agonists):
http://www.ncbi.nlm.nih.gov/books/NBK64236/
3) some explanation per the specifics of the mu receptor (click on this only if you are truly thirsting for knowledge on the mu receptor):
http://www.iuphar-db.org/DATABASE/ObjectDisplayForward?objec...
Have a nice night!