Sam,

If you can get hand on "Oracle Security Handbook" (Osborne ORACLE Press Series) by Marlene L. Theriault - it has a very detailed description how OS authentication works, step by step. Separately for Unix and for Win$, as the process of user authentication differs substantialy between those two OS. I don't have the book handy, but I remember it describes this topic extremely understandable and in great detail.