No, you won't lose your book. Remember, in addition to the Purchased list on "My Library", you also have a purchase history under "My Account", if for some reason a book you were to delete from the desktop or a device or app no longer showed up in Purchased or Trash in the library, and you could not restore it using other means, Customer Care can verify your prior purchase and make it happen.
FWIW, I routinely delete books from my Touch after reading them. I have never lost one
.