1. A .deb is simply a tarball. If you didn't know that, then you could simply download the package's source tarball.

2. The distribution of Debian using the new installer is not released yet. Once it is released, the documentation will be placed on the main web site.

3. If you really want to read the installation guide for the old installer even with the upcoming release pending, you could go to the amazingly hard to find link http://www.debian.org/doc [debian.org] and click on "Debian Installation Manual".