I used the grimoire by accident and now I presume I can't complete the quest. Do I have to return to the last checkpoint and lose ALL that I did in order to have it again?
You can find The Black Cat in Gran Soren. There, the shopkeeper offers you the chance to make forgeries of items, which will never inherit the magical properties of the original. They explicitly state:
If Salomet's Grimoire is used, gifted to a pawn, or lost, a forgery becomes available.
For more information, check out The Black Cat entry on the wiki.
It's also good to note that with the exception of 2 (The Grimoire being 1) items, lost quest items will also appear in this location.