NeXT was a truly innovative platform, actually favored by traders, and many specialist aplications were developed for it. Nowadays we have OpenStep and Mac OS X, which probably is the most elegant looking and designed operating system out there. It is based on the Mach microkernel, and is in some respects compliant with UNIX systems.
When really wanting to use the UNIX advantages and powertools, one has to be able to use the shells efficiently.
The best book I read on the subject was : "UNIX for the impatient".
I actually have both the 1st edition and the second edition. One of the strongest reasons for having this book ready on your desk, is the very logical and strong reference throughout the book. The 2nd edition also includes the book electronically on a CD.
Many of the UNIX strongpoints are also available under Windows using Cygwin etc., but it will never come close to the extreme reliablilty of UNIXes. I've administered many flavours including SunOS, Solaris, HP/UX, AIX, SGI Irix, Linux and others.