You can download the source code for GNU utilities from
www.gnu.org, the kernel from kernel.org, and source code for various other utilities from sourceforge.net and freshmeat.net.
If you are just talking about command usage, there are *many* good books available - Linux in a Nutshell from O'Reilly is probably your best choice for an all-in-one command reference covering 90% of what you'll need day-to-day.
Cheers
ZB