Skillbooks like this one are designed to utilize the "Special" (also referred to as "Skill") of the shown Character. Use these books to "Power Up" a unit that has the same "Special" name as the skillbook.
There are two possible outcomes after feeding a skillbook: 

• You are lucky and get 5000 XP AND a Skill Level-Up.

or

• You just get the 5000 XP.

The number of skillbooks you use at a time does not matter. Using 1 skillbook could get you a skill up. Using 5 skillbooks could get you 5 skill ups or even none! It is all RNG (Random Number Generator) based. 

To raise your odds of getting the skill ups you want, it is best to store your skillbooks until there is a "2x Skill Up" event. This event is usually held once a month.

Skillbooks that are not needed can be used as 5000 XP for ANY character you wish.

The following is a table of all Fortnight Islands as well as the skillbooks available in each. This is to assist with planning your fruits so you can max any characters you desire.