There are a wide range of resources available for Linux online. The following links provide some overviews of commands and packages used in this book:
Screen user's manual: http://www.gnu.org/software/screen/manual/screen.html
Raspbian package information: http://elinux.org/Raspbian
apt-get user manual: http://linux.die.net/man/8/apt-get
Wget user manual: http://www.gnu.org/software/wget/manual/wget.html
Linux Kernel Archive: http://www.kernel.org/
Geany IDE: http://www.geany.org/
Make command manual: http://linux.die.net/man/1/make
Chmod manual page: http://linux.die.net/man/1/chmod
Chown manual page: http://linux.die.net/man/1/chown