I've never tried in CS5 or later, but I don't see why it wouldn't be possible, though you might not be able to do things that require modifying the already open file.
Peter Spier I see your point but in this case why opening a Book will try to modified its document. And hence no menu item or other user interface to do so. I mean a Book doesn't modify the InDesign document when opened via the Book panel (I tried and didn't see any * mark in doc title which shows that doc has been modified). And its possible to open a Book no matter if any of its document is already opened and modified. So I believe it is quite possible(although I can't be sure as its folks at Adobe who knows better) but I don't any see UI. May be Adobe think its of less/no significance to open the Book via its document rather its more meaningful to open documents via Book.
As far as I know, there is no way to open a Book from one of its component documents, only from the File > Open... command or by double-clicking the .indb file in Finder/Explorer/Bridge.
Since I work alone and all of my files are stored on my hard drive my testing may not be valid for networked workgroup situations, but if I open a file and then open a .indb file that references the open file it simply shows the file is open in the panel, and since I'm the user who opened the file originally I can edit it. I don't know what would happen if some other user had a file open and I tried to open a ,indb file that contained it. As far as I can tell, there is nothing preventing multiple users from opening the same .indb file simultaneously, either.