The Fine Print: scanned texts
Yes, it is possible to copyright scanned texts, even where copyright for the printed version has expired. If it were not, nobody would bother to scan any of these olde ones. Because in that world, the minute you'd put your hundreds of hours worth of scanning, OCR'ing and html'ing online, some busybody would copy it over to his/her site. And/or flog a CD of it. Without even so much as a by your leave. Because they'd be legally entitled to, right? Funny how right and wrong never seems to enter into it.
Talking of legally entitled: Paul Bergner has the full text of King's American Dispensatory (from this site) on his MH Resource CD. That CD also features the full text of Paul Bergner's excellent journal Medical Herbalism, and his scan of Cook's Physiomedical Dispensatory: http://www.medherb.com/full_text.htm. It's excellent value for your money.
Back to those entrepreneurs. I haven't scanned these works for busybodies to make money off them; I've scanned them because I want to, and because I like the full-text searchability of them. If you want the full text of one or the other classic text on your site, why, scan one. Try anything by the Lloyd brothers, or by Ellingwood. Or Hager's Handbuch, or other similar large old pharmaceutical works, in any of the languages you speak. Or one or the other journal that is repeatedly mentioned in King's. The best years are from 1880 through to 1930 or so. Really, there's lots of good herbal books at your local antiquarian's, or over at http://abebooks.com or http://bibliofind.com. Send me the URL after you've gotten a good start (say, 100 pages or so online), and I'll link to you. If that's not feasible, well, you may copy a few pages (less than ten) of the larger works to your site, but if you do, you have to link back to this page from every page you've taken. The URL to link to is http://www.henriettes-herb.com/eclectic .