READ_ME A description of what is where and how to compile RELEASE_NOTES A history of how sendmail reached this point FAQ A compendium of frequently asked questions doc/op/op.ps The Operations Guide in PostScript
The READ_ME file contains information about all the other parts of the distribution. It describes how to format the manual pages, explains which RFCs are important, and anticipates some problems you may encounter when building.
The RELEASE_NOTES file describes the history of sendmail in far more detail than you may ever need. The first part of this file lists important late-breaking news about the program. New features will be listed here, and critical bug fixes will be described.
The FAQ (Frequently Asked Questions) file is a copy of the one that is distributed in the Usenet newsgroup comp.mail.sendmail. Many people have already asked the very same questions you probably have percolating in your head. This document tries to anticipate the most common of those questions and to provide answers to them.
The doc/op/op.ps file is written by Eric Allman and is entitled SENDMAIL Installation and Operations Guide. It is the final and authoritative word on all matters relating to sendmail. It is dense reading, and not suited for the beginner, but well worth perusing in this early stage of learning sendmail. Unlike the first three files (which are directly readable text), op.ps is PostScript. 
 The source for the PostScript is in op.me and requires the latest tmac.e macros to format.