IANAL.
Legally, you're in limbo. Let's take the closest analogy: making a cassette tape copy of a CD you own. As near as i can tell, the RIAA thinks that's illegal copying, but not worth the effort to prosecute. The court rulings i'm aware of (such as the one making VCRs legal) seem to imply that the RIAA is right: if you buy one copy of a work, and end up with two copies (through personal duplication), you've done something wrong. Now, where it really gets fuzzy is the sometimes-legal proposition of archival copies. In many cases (software installer CDs come immediately to mind) it is perfectly legal to make a copy for backup purposes only--IOW, in case your original is lost or destroyed. The implication is taht at no time will you have more than one "functional" copy--because the backup is locked away until you no longer have the original. Now, i suppose you could argue that you never use the hardcopy and the digital copy at the same time (assuming that's true), and so it's no different from making a cassette copy to listen in the car, but using the original CD at home (and they aren't both used at the same time). But, remember, other than software installers (and works owned by a library), i don't know that this is ever legal--it may be just a case of the copyright holders turning a blind eye, or not being able to functionally do anything about it.
But, if you want two functional copies, we'll have to turn to Fair Use, IMHO. Fair Use certainly allows you to make a single copy of a single page of a book to use in addition to the book, even simultaneously--such as so that you could have the book sitting open to one page, and still have the other page accessible. Fair Use certainly does not allow you to give copies to others, to make multiple copies, or to copy the entire work [with some exceptions for educational institutions--and, no, teaching your friends Midnight doesn't count
]. Anything between those two extremes is pretty much up in the air--though the closer it gets to the former, the more likely it is legal, and closer it gets to one of the latter, the less likely.
So, to your particular situation. My interpretation is that you may scan your book and use the digital copy, even if you still have the hardcopy. You may not loan one (say, the book) to a friend while still using the other (in this case, the PDF). You may not download someone else's digital scan of the book--while the end result is the same as if you had scanned your own copy, the means make it nonetheless illegal, because you are participating in illegal copying.
That's legally. Morally, i'd say it's fine
provided the following things are true:
- you own the book (as you've already said)
- you do not use both copies simultaneously--no fair loaning your friend the hardcopy because you can just use the digital one
- FFG is not now distributing a commercial digital copy
- should FFG start distributing a commercial digital copy, you either buy the legal digital copy, or destroy your illegal digital copy
- you do not distribute the digital copy--you can't control what others do, but you can make sure that you're not facillitating illegal/immoral activity
