No, you cannot sell the Herbal. But it is possible to destroy it in your inventory. The book should drop from Orc shaman Revkan in the surface part of the Iron Mines. He should be somewhere near that tent on the right of the map.

Thanks for your reply. I killed all orcs in Iron Mines but I can't find the book. Can it be a bug or something like that?

No, I don't think this is a bug. If you kill Revkan, the book should drop. I don't recall any bug of this kind in Inquisitor. But don't worry, you can always use a cheat to get the book.

Just press ~ (next to 1) to open the console and type this command:
\givenewitem Herbar
Press Enter and the book should appear in your inventory.