[Nook HD, HD+ General] wayover13:
I managed to fix my Nook HD+, which had gotten itself stuck in some sort of boot loop, by installing LineageOS as discussed in more recent posts in this thread. I had to sort of cobble together, from a couple of threads (including this one), the steps I would need to take in order to execute the process successfully. A bit of translation of concepts was needed since most directives for doing this sort of task are geared toward Windows users, whereas I am a GNU/Linux user. I ran into an undocumented glitch toward the end, but that was easily surmounted. I will now list the steps I took to accomplish the task.

To begin with, I more or less skipped an essential first step since I had an SD card laying around that I'd previously used for installing CWM on my Nook. Had I been starting from scratch, I assume I'd at least had to have set the SD card's partition to be bootable, which I almost surely would have done using the cfdisk program. Some directives have you create a FAT32 filesystem on the SD card but, so far as I'm aware, these cards already come with a FAT32 filesystem on them. As the case may be, my card already had the needed filesystem and the boot flag was already toggled for its only partition.

Next, I downloaded files I would put on the SD card. The names of those files were sdcard-reco-ovation.img.xz, lnos_ovation-ota-NJH47F.171021.zip, twrp-3.0.1-0-ovation.zip, and open_gapps-arm-7.1-pico-20171228.zip and they were found at the link provided earlier in this thread by PeteInSequim and by digixmax in the thread over at https://xdaforums.com/nook-hd/general/how-to-installing-ln14-1-internally-hd-t3562810.

In order to put needed files from sdcard-reco-ovation.img.xz onto the SD card, I first had to uncompress the archive, which I did by running tar xf sdcard-reco-ovation.img.xz. That left me with the uncompressed image file sdcard-reco-ovation.img.

To mount that file as a looped filesystem from which I could copy contents to some other location, I had to first determine the offset of the filesystem contained in the image. The command sudo fdisk -l /path/to/sdcard-reco-ovation.img gave me necessary information for doing that: it showed sectors in the image's filesystem were 512 bytes, and that the start point of the image's filesystem was 59. Thus, I used the command sudo mount -t auto -o loop,offset=$((59*512)) /path/to/sdcard-reco-ovation.img /mnt/loop in order to mount it.

At that point, having already mounted the SD card, I was able to copy over from the loop-mounted image, needed files to the SD card. I followed directives digixmax inidicated that he'd given in steps 1-3 at https://xdaforums.com/showpost.php?p=51043572&postcount=1.

Once I'd copied those over from the loop-mounted image file to the SD, I proceeded to copy over to it the other files digixmax lists at https://xdaforums.com/nook-hd/general/how-to-installing-ln14-1-internally-hd-t3562810. I then followed his further directives in that post, found in steps 3-9, in order to initiate the LineageOS installation.

The glitch I experienced was that, on rebooting the device at step 9, it seemed to get stuck at the Nook boot screen and would not proceed further. I decided I should manually power off the device and reboot to see what would happen. Once I'd done that, the Nook boot screen soon gave way to the Cyanoboot screen and the device booted up as expected. The little bit of testing I've done seems to indicate that the device is back to good health.

I have a couple of questions about restoring from back-up that I'll ask about in a subsequent entry in this thread. Thanks for the helpful tips provided here by PeteInSequim and digixmax and for their assistance in aiding us in keeping our Nooks useful beyond their intended lifespan.