What are SBs ?
Skill Books, also known as SBs, are used to level up your skill once it's M [master] (See: M Skills).

How to make my skill Master ?
In order to master your skill, you need to get it to at least 17 level.
Depending on your luck, it may master on 17, 18 ,19 or 20 points (on 20 it always masters).

It is important to note that one should always master skills at 17. You have to focus on just one skill to master each time, and leave the other skills at 1 point or even 0.

How do i gain Skill Points ?
Each time you level up, you gain 1 skill point.
A simple formula to see your total skill points : LVL-1=TOTAL SKILL POINTS

I'm at 17/18/19 points and didn't master my skill ! How can i get my skill points back !?
The simplest way is to use Tomes of Forgetfulness. Then you re-add your points to the skill [it may still fail to master it, and you will have to try again].
You can also use "Skill Reset Scroll", from item mall. Side note: You will receive up to 17 points, regardless of whether you have put more points on the skill, and you cannot get lost points back.
NOTE: If you are 30 or lower level you can talk to Old Lady. Pay some money, and she will reset both your skills & force. The required amount of money increases on as you level up.

I keep reading SBs, but they are failing!
SBs can fail. They only have a ~30% success chance. (That means, for every skill you want to get to G1, you have to read a total of about ~183 SBs/ ~6 months if you read a SB every day).
You can use "Tomes" instead of SBs, it is thought that they offer a higher success rate. They can be mainly found from Golden Treasure Boxes+ and are relatively rare.
You need to successfully read as many SBs as the current master lvl, as shown below :
Lvl1: 1 SB
Lvl2: 2 SBs
Lvl3: 3SBs
Lvl4: 4SBs
Lvl5: 5Sbs
Lvl6: 6SBs
Lvl7: 7SBs
Lvl8: 8SBs
Lvl9: 9Sbs
Lvl10 to G1: 10SBs (Total: 55 successfully read books)

NOTE: You need to wait 16-32 hours between a SB and another.
You can use item mall to read a SB before 24hours, or to get 2.5x times higher success chance.
NOTE2: You CAN'T make a skill G at M7 , even if the descriptions says it's possible.

My skill is G ! What do i do now ?
Now your skill is G [Grand Master] (See : G Skills), you need Soul Stones ! They can be dropped by Bosses, as well as from Metin Stones of level 40 and above. Soul Stones have ~30% chance too. Unlike SBs, you need only one successful use of a Soul Stone for each level up.
Each Soul Stone takes some of your HKP (Hostile Killer Points] off. Beware, players with Negative HKP at 50 level or above have chances to drop items from both their inventory and equipped items, when they die (monsters do not pick your items though). Only an equipped Ring of Lucy can prevent items from being dropped, by losing this instead of them.

After G10, it's P [perfect] skills (See: P Skills). It's the max! Congratulations!

How do I obtain SBs ?
SBs drop from low and middle level Metins, mostly.
They may also drop from the Demon King. The SB dropped is random, although the lower lvl you are, and the higher lvl the Metin (see: Metins) is, you may obtain more SBs. [3 SBs from 4 Metins, instead of 1/4 (this is only an example)]. Conversely, if you surpass Metin level more than 10 levels, you shouldn't expect them to drop anything special.