CSSU is Community Seamless Software Update, operating system upgrade provided by community, it fixes many bugs not fixed by Nokia and adds some usefull stuff, see http://wiki.maemo.org/Diablo_Community_Project and do as told in "Upgrading to the stable version" section.

You may also want to install diablo-turbo kernel (see this thread for information what it is, there is also a link to page where you can read how to install it).

Maemo is based on Debian, so if you try to find some literature, you may search Debian.

For writing scripts that are run at startup, you may want to read this page. However, keep in mind that if you want to run GUI application (GUI means Graphical User Interface - so it's a normal app with windows, buttons, etc.) it may not be the best place for it (because /etc/init.d scripts may be run before the window system has started, as far as I know, and GUI program needs window system - which in Maemo is called Xorg and Hildon - running).

Good luck!