For what it's worth, I think "outliner" is an absolutely horrible name. An "outliner" software *means* something, and they'll expect the usage of that name to mean what they expect (which is why "category" was such a horrible choice for "taxonomy") - it'd be like an OPML editor (like Dave Winer's) or OmniOutliner. If you'd like to try out a crossplatform (Java) opensource outliner, that mimicks what people expect an "outliner" will do, check out the screens here: http://outliner.sourceforge.net/.
In short, our book.module is absolutely nothing like an outliner. An outliner needs to be *quick*. Our book module requires about 3 million more clicks than an outliner-expectant user is going to put up with.
Of course the renaming should be companied with reworking of the UI, introducing better outlinging functionality. The potential is in there, but the interaction patterns are not fortunate... It is not suitable now for non-geeks even to author books effectively (I know I have been preparing an S5 slideshow with book module these past days :). Goba