from what little (very little) i know, Ralph Sir Edward is right ; copyright law is far too complex and the answer varies from country to country.
however many people feel that such cases *do* fall under "fair use" as long as you are doing the scanning for your own personal use and are not intending to sell or otherwise distribute it. it's considered comparable to ripping a cd you have bought to mp3 files so you can listen to it on your ipod. this is called "format shifting". it does often come down to "do what you think is ethically acceptable".
also, don't forget that while the legal status of this activity may be unclear in some countries (in some countries it is explicitly legal, i believe in canada for instance although i could be wrong), in order to be prosecuted for it even IF it were defined as definitely illegal, you would have to get caught. i'm not really sure how someone quietly scanning copies of their own legally-owned paper books for their own personal use, not uploading them anywhere, could ever possibly come to the attention of any legal entity.
personally, i happen to consider that a completely fair use of any media that i buy ; if i've bought it, i think i should be able to read it however i choose.
EDIT : i see wallcraft was faster than me and also better documented.
