Here is the new manual. It is in Docbook XML format, which is
becoming the new standard for documentation. The advantages include
more precise tagging, figure and table support, and a very strongly
supported toolchain. There is a fairly brief cheatsheet.xml here which
will probably tell you all you need to know, especially if you've
worked with debiandoc before. The Definitive Guide to DocBook by Norm
Walsh is indispensable for figuring out why your tags don't work the
way you think they should:

http://www.oreilly.com/catalog/docbook/chapter/book/docbook.html

The sources can be found at
https://salsa.debian.org/installer-team/installation-guide
Checkout with:
git clone git@salsa.debian.org:installer-team/installation-guide.git

For more information about the manual, see:
http://d-i.debian.org/manual/

See also the various files in the subdirectory doc.

All help is welcome! Send suggestions to the debian-boot list. Patches
can be submitted against the package installation-guide.

Patches should normally be submitted against the 'trunk' version of
the manual, so they will be included in the next stable release.
However there are some cases where it is appropriate to update the
stable release also: such patches should be made against the branch
for the relevant stable release codename: ^/branches/manual/<codename>.

Last update: 03 June 2018