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.