Debian New Maintainers Guide - Good resource for learning to package.

Debian Policy - The essential Policy manual for Debian and Debian-based distros.

Debian Developer's Reference - Specific information for Debian Developers but has some items of interest for packagers.

Library Packaging Guide - Guide for packaging libraries.

Debian Women Packaging Tutorial - Another good introduction to Debian packaging.

IBM Packaging Tutorial

Duckcorp CDBS Documentation

Ubuntu MOTU Documentation

Kubuntu Packaging Guide