I'm not the person to answer DRM questions, because I vote with my wallet (tiny though it may be) and refuse to buy DRM-restricted ebooks. I don't think DRM restrictions are "fine" so I buy from Baen, from BookView Cafe, and a few other random places, but not the Sony store. I also read a
lot of free ebooks. Project Gutenberg has been digitizing books since the 1970s, believe it or not, and they have tens of thousands of them. I'm a little notorious for posting my great ebook source list at the slightest excuse -- you'll find it further down in this post.
Calibre can handle any number of reading devices just fine. There's no reason not to share a library, unless you've got DRM stuck all over your books. Calibre, by the way, is free. Really free. The GPL kind of free. No accounts, no worries. And it does pretty much anything you can imagine. If you and your sister want partially-overlapping libraries (that is, if you like books by Austen and Baum, and your sister likes Baum and Cooper, each of you can assign them tags in calibre ("mine" and "hers", perhaps) and look only at the books that are tagged "hers" but not "mine", for instance. Drop in on the calibre board (it's down the forum list a ways) and ask any questions you may have, and the person who wrote whatever feature you're asking about will probably answer your question himself, unless the user-gurus beat him to it.
External chargers are better than USB because they're faster, they don't interfere with reading your books (when you plug a USB cable into a Sony Reader, it expects books and goes into transfer mode, even if it's just a USB power brick), and they're the only thing that can charge a Reader with a totally flat battery. I have an actual Sony charger, and the thing is rated at 2 amps. For such a little device, my 505 can be awfully hungry! Incidentally, do not plug the Reader into an unpowered USB hub, or it will try to power the hub and drain its battery.
Now, about where you can get ebooks by the tens of thousands:
Right here on MobileRead is a good start -- the ebooks are built by hand by MobileRead members, and in my opinion they have the best formatting. There are a lot of other good ebook sites out there. Here's my personal free ebook source list:
Public Domain
MobileRead - best formatting, limited selection (hand-built by MobileRead members)
ManyBooks - PG scraper, but sometimes has books from other sources, good formatting
Feedbooks - mostly scrapes PG, improves formatting
Project Gutenberg - the granddaddy of them all, phenomenal selection
Munseys - painful to use, but has books nobody else does
the Internet Archive - generally only the scans are readable
Google Books - most of the time, what's good isn't free, and what's free isn't good
Free non-PD
Baen Free Library - amazing selection; also, they
will tempt you into buying books
5th Imperium - Baen CD collections
Other Things
Search:
Inkmesh. Sadly, it's not a very comprehensive search engine, at least when it comes to free ebooks - it misses a lot of them - but as far as I know, it's the only ebook-specific one out there.
If you're using free ebooks, it's only right to give something back: proofread a page a day at the
Distributed Proofreading Project.