Life With Unix : A Guide for Everyone.
This is one of my all time favorites - it's the "other book" about Unix. Written in 1989 by Don Libes and Sandy Ressler it covers Unix history and trivia like why the biff command is called biff (not that biff is in wide use these days). From "The Future of Usenet" in the UNIX Underground section: "Usenet is also feeling pressure to become more commercial". How true - I can still remember the first day I saw a .com email address (most people had .edu or .gov or country addressess like .no back in '89). Buy one if you can get hold of it (rare and thus expensive - between$80 and $299 for a used copy when I looked at amazon in November 2002).
If you were going to buy just one book for managing Unix the clear winner would be this book by Nemeth/Snyder/Seebass&Hein. It's very concise and deals superbly with the differences between 4 major versions of Unix (Solaris, HP-UX, FreeBSD and RedHat Linux). It excels at showing someone who has good knowledge of one Unix dialect how to do tasks (like add a disk) on another Unix.
There are extensive chapters on sendmail and DNS/BIND. Buy this book and save money on not needing the O'Reilly sendmail book. Configuring sendmail does not need to be that hard or cryptic: use m4 macros and make sendmail almost easy (RedHat 8 ships with an uneccessary complicated sendmail.mc file). With this book and the README distributed with sendmail you will have a custom sendmail configuration up and running in no time (well at least compared with the old days). It last came up trumps for me when Sun upgraded sendmail to 8.11; there's a good explanation (with a figure) on the split between transport (port 25) and submission agents (port 587). USAH also covers DNS in a good way - but I would still urge you to get the DNS book from O'Reilly: it's another an almost perfect piece of technical writing. A full review of the handbook is available on amazon.
Every Unix sysadmin should spend some time reading "real" books. In fact Martin Vermeer suggests there is a connection between Unix and literacy. So why not pick up the book from where the operating system got it's name (a search on google news could give me no proof of this - once I have some hard evidence I will provide it). Solaris is an excellent sci-fi/psychological read and was later filmed by Tarkovsky (and has now been remade by Stephen Soderbergh with George Clooney as Kelvin).