Click the little arrow that appears in the top left of the frame surrounding the book in the library view, then select "delete" from the menu that pops up (on XP anyway, not sure about other OS's)
This may still leave the book in a folder in your computer ("My Digital Editions" in "My Documents" in XP so you may want to check there if you want to file removed from your disk as well.
Cheers,
Bob