the debian policy manual describes how debian packages are to be prepared, http://www.debian.org/doc/devel-manuals#policy
unlike the packaging manual which is just 'how to build a package' it covers a lot of the design of debian as a system, broken down into requirements on the individual packages.
there's also lots of user documentation: http://www.debian.org/doc/user-manuals
, if you're the sort who really likes to read.
debian installation guide and debian reference are excellent