Recommended references

The Practice of System and Network Administration by Thomas A. Limoncelli and Christine Hogan
The Art of UNIX Programming by Eric S. Raymond
Programming Perl, Third Edition by Larry Wall, Tom Christiansen, and Jon Orwant
Perl for System Administration by David N. Blank-Edelman
Absolute BSD: The Ultimate Guide to FreeBSD by Michael Lucas
The Official Samba-3 HOWTO and Reference Guide by John H. Terpstra and Jelmer R. Vernooij
The Book of Postfix by Ralf Hildebrandt and Patrick Koetter