You can destroy the book if you do not want to be PvP.
If you give him the book, you will be flagged PvP and not be able to get buffs or beneficial spells from others who aren't also flagged. The PvP population on blue server is extremely low, so it's not really worth flagging yourself unless it's for a specific event. (Usually if there's an event, a GM/Guide may be there helping out who can flag and unflag you without a book.)
You can get another book from the Priest of Discord at any time. A few seconds of chatting with him will get you what you want, if that's what you want.
To get unflagged without GM or guide intervention, you need to travel to Qeynos and speak to the Priest of Order who wanders around the newbie area and just inside the city.
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