@Jack79 There is no hard limit on the number of pages you can copy and still be able to apply a Fair Use defense. One famous case held that even 300 words copied from a 500-page book was not Fair Use, in part because those were the 300 most important words. For some works, you can use every page, and for some works, you can only use a handful of pages. Wherever you read that 30 page limit was laughably inaccurate.
@rottenit The DMCA has nothing to do with this, and you’re entirely wrong about memorization being a DMCA violation. I’ll assume you have a very dry sense of humor.
To answer the original question, yes. You can have personal backups of books just like with CDs and DVDs. (Technically, under the DMCA, backups of CDs and DVDs are on shaky ground, but nobody’s ever been busted for making personal backups. And the DMCA does not apply to traditional dead-tree books.)
Let’s go through three ways in which you’d get a “backup” copy of this book on your Nook.
(1) It’s not illegal to copy the book into a PDF by yourself and then upload it to your Nook. Personal use (at least for books) is strongly favored by American copyright law, and practically speaking, no one else would ever even know, because it would only be between you and your Nook.
(2) It’s arguably not illegal to download the book in PDF form from a website and upload it to your Nook. Again, this is pretty personal use, although if the relevant authorities ever got ahold of that server’s logs, they might make your life rather miserable. You’d be hard pressed to prove that you already owned the book at the time, but assuming you could, you’d be pretty safe.
(3) Torrents are a special case, because you’re simultaneously downloading and uploading the file all around the world. In this case, you’re making way more copies than you’re allowed to: you’re not making complete copies for everyone in the swarm, but you’re certainly making copies of bits that get uploaded elsewhere. This is a Very Bad Thing: there are practically no circumstances in which you’d be allowed to make copies for strangers in this way.
Assuming you only leeched from the swarm, you would be back on the second use case (downloading from a website): still not super strong protection for your actions, but you could theoretically “win” after a lengthy trial. I say “win” because you’ll spend an awful lot on lawyers, and probably just settle before an actual trial happens. Of course, the odds of you being busted for torrenting anything are astronomically low to begin with.
I am not a lawyer, and this is not legal advice. I’m a law student who has done well in Copyright class, but that’s about it. See an actual lawyer for actual legal advice.