If you picked up one of the readily available rune sets in a book store, chances are incredibly good that you got a set with 25 runes, the 24 runes of the Elder Futhark and an additional empty stone - the blank rune, Odins' rune, Wyrd rune or Rune of Destiny.
What this basically means is that the author doesn't have a clue and that he mostly copied from the works of other authors before him. There is NO historical evidence whatsoever that the Norse used a blank stave in divination or magick. Additionally, the power lies in the symbol not in the carrier.
One of the first offenders is Ralph Blum. His Book of Runes and all the subsequent titles have been immensely popular in the 80s and sadly the blank rune is now here to stay.
Those that do no actual research integrate the 25 runes system in their works, most notably are the various Wiccan groups who adapt almost everything for their religion - one of their mottos must be "Let No God Be Left Behind". Not to mention that the lore gets watered down to no end. It's now all about love, peace and no worries or troubles. While many of the runes promote peace, the experienced runester knows that there can be no growth without struggle.
My suggestion is if you get or have one of those books, keep the set and throw away the book. The empty stone can be used if you lose one of the other runes.