To get started with Android for Nook HD/HD+, you'll need to get familiar with Git and Repo.
To initialize your local repository, use a command like this:
repo init -u git://github.com/airend/android.git -b nougat
Then to sync up:
repo sync
Choose a target:
lunch aosp_[ovation|hummingbird]-[eng|userdebug|user]
Please refer to the AOSP Guide for further building instructions.