Fairy Mastery

Getting this book is pretty tricky because from what I could tell, you need to be doing the Everling Locket quest to be able to get access to the area of the castle that the book is located in.

Location 1: Nektulos Forest

First you need to get to Nektropos Castle. To do this you must have completed the mastery quest for it. Head to the northeast area of the zone to get to Nektropos Castle.

Location 2: Nektropos Castle

Now, in order to get the Fairy book, you will have to know your way around the place and be doing the Everling Locket quest, to where you gain access to the secret tunnels which connect all of their bedrooms. You must be at the stage of the quest where you have killed the 6 sisters. This is the point where the ghost helping you will unlock the doors in the castle so you can move around. Each of the sisters rooms have a door in them which leads to a series of secret tunnels which link to all the sisters rooms.

Go to the last sisters room, which is the one that spawns the level 40ish triple arrow monster and go through the door in her wall. The monster might attack you, but he cannot move, so just run past him and make sure not to engage him.

Inside these tunnels just walk in and go around the first corner and you will see a pair of double doors which lead into the library. You must obtain the library key or be grouped with someone who has the key in order to get inside.

Once inside there is a bookcase which opens up like a door.

Enter this door and there will be a desk to your right. Examine the book on top of it to get the Fairy Mastery book.