dtracebook.com is currently offline. It hosted the scripts from the DTrace book, which you can download here: dtbook_scripts.tar.gz.
On this site you can also find my original DTrace Tools page and the DTraceToolkit page. There are no plans to update or maintain these. The wonderful FreeBSD folk are now maintaining updated versions of the DTraceToolkit on github, here: github.com/opendtrace/toolkit.