Originally Posted by Andreaz
You use the feat to write the spell down in your book. If that book is gone, you lost that copy of the spell. If you don't have another copy, you don't have that spell anymore.
Well yes you'd loose it so much as you loose magic missile. You just need to find a scroll of it and bam your back in business.
Also you can make that spell one of those you remember from memory spells (I can't name the feat at the moment).