Sendmail, 4th ed., by Bryan Costales, Claus Assmann, George Jansen and Gregory Shapiro (O'Reilly, 2007).
The latest edition of the "Bat Book"--the most authoritative source of information on sendmail. Updated to cover sendmail versions through 8.14. This is a "must have" for any serious sendmail administrator.
sendmail Performance Tuning, by Nick Christenson (Pearson Education, 2002).
If a high-performance, high-volume mail server is your goal, this is the book for you! Based on sendmail 8.12.
sendmail Milters: A Guide for Fighting Spam, by Bryan Costales and Marcia Flynt (Addison, Wesley 2005).
Before discussing mail filters (milters), the authors spend significant time describing the history and characteristics of spam. They then talk about test environments, the milter API and "nuts and bolts." The book is a good read--even if you don't want to write your own milter! Based on sendmail 8.12.
sendmail 8.13 Companion, by Bryan Costales with Gregory Neil Shapiro and Claus Assmann (O'Reilly, 2004).
You won't need this book if you own the 4th edition of the "Bat Book" (above) This was written to update the 3rd edition of that book, which has now been superseded.
Sendmail Cookbook, by Craig Hunt (O'Reilly, 2003).
Although becoming "dated," you might still be able to find one of its recipes to make sendmail do what you want.
Linux Sendmail Administration, by Craig Hunt (Sybex, 2001).
Once a useful book, this title is probably no longer worth your time and money (unless you can purchase if used for a few dollars). After reading it as a tutorial, you should probably supplement its contents with the latest edition of the Bat Book (above). This title is based on sendmail 8.11.
Sendmail: Theory and Practice, 2nd ed., by Paul A. Vixie, Frederick M. Avolio (Digital Press, 2002).
Do not buy this book if you are interested in learning how to generate a "modern" file or how virtusertable works--it doesn't include current information on features added to sendmail since the late 1990's. Purchase this book only if you are interested in a historical view of sendmail as it existed in the 1990's. It includes in-depth discussions of (now) seldom-used UUCP and DECnet, as well as coverage of old mail transfer and user agents (but no mention of the more recent Qmail, Postfix or Exim).


SSL and TLS: Designing and Building Secure Systems, by Eric Rescorla (Addison-Wesley, c2001).
A thorough and understandable treatment of Secure Socket Layer/Transport Layer Security.
Network Security with OpenSSL, by John Viega, Matt Messsier, Pravir Chandra  (O'Reilly, 2002).
Focuses on OpenSSL's implementation of SSL/TLS. (OpenSSL is used by Apache Web Server, sendmail, and numerous other Open Source programs.)
SSL & TLS Essentials: Securing the Web, by Stephen A. Thomas (Wiley, c2000).
A quick introduction to SSL and TLS.
Applied Cryptography, Second Edition: Protocols, Algorithms, and Source Code in C, by Bruce Schneier,  (Wiley, 1996).
Everything you want to know about the underlying technologies used by SSL and TLS. Thorough, well-written, understandable.