Depending on which Nook you have you'll have different names for the same option.
On the Nook Color and Nook Tablet, in your Library press and hold on a book. You'll have a drop down menu with the options 'archive' and 'delete'. Delete will remove the book from your account, if you want it again you'll need to repurchase it. Archive does what you want.
On the Nook Simple Touch (both regular and Glowlight), open the book, tap in the middle of the page, select 'more...' and you'll see similar options.
The Nook HD works like the Nook Color and Tablet, except it calls archiving 'Move the the Nook Cloud' or something along those lines.
It is worth noting that on the HD the book will visably stay on your screen, but will have a cloud with a green arrow in it in the top left corner. I believe there is some way to archive it off the screen (probably by archiving it from BN.com). However it's not taking up space at this point.
Delete - on all devices will mean you need to repurchase the item if you want it again.
Archive / Move to Nook Cloud - will remove the file from your Nook, but keeps it on your account so you don't need to repurchase it.
on the nook hd press and hold to volume up button then press and hold on the book then the archive option will show up.
If i have read books, what do I do with them? Cab I delete them off my nook and will they still be available through BN for downloading? I have an incredible number of books on my Nook and would like to get rid of some of the books as I read them.
Also I was wondering about exporting them to my laptop for storage, but did not know the file where nook books are stored on my nook...as I looked at the files and they all look empty.
You don't say which NOOK model you own, but these instructions should apply to most of them:
To remove a book from your NOOK but still have it on your online/cloud library - you need to archive it either from the NOOK or the B&N website. There should be an option on your NOOK to not display archived books.
The best way to save a copy of your books on your home computer/laptop is to install the NOOK app for PC or Mac and use that to download your books.
When you do this, make a note of the credit card number that is the default on your account at the time of download, as this might be required to unlock the books at a future date if B&N is no longer around, or you choose to go with a different brand reader.