[Pajamaman]

I guess you mean the nook glowlight 3 7.8". You'll have to root it, install a launcher (Relaunchx), the add koreader.

You can find instructions on how to do this on the xda forums. Google "xda nook glowlight 3 root". Note, the instructions are for the 6" model. I'm not sure if you'd if need to change anything for the 7.8".

Personally I found Koreader to be unstable to the point of useless on Android, but maybe that was just me.

Alternately, you could install another ereader software like alreader for example. A copy of fbreader compiled to work with buttons is available on these forums.

Take a look at that xda thread and see if you think you can do it. The software on the nook is a bit flaky. You need to tread carefully. I did manage to brick one, but that was because of a bug that is now known and documented on that thread.

I have ordered a new nook and will be rooting it in the near future, so I reasonably sure I can do it successfully. Whether you are confident about doing it is your call.