So here are the parts necessary for the 5 Rain skills in my mod Unfair Difficulty-
http://www.mediafire.com/download/42e86dpsf5roeco/Rain_Skills_Folder.zip

They're not in a mod of their own per se. The parts included in that zip can be dropped into the appropriate folders of a different mod and have their effect.

Something that might not be obvious is the structure of the lines within the Links folder of the Stats.

Like for example, I have this for my Rain skills-
Code
object itemstat "SKILLBOOK_UNFAIR_Rain_Blood","2c7da171-f6c1-4df7-bd3f-7136931ce04a","",0,0,"SkillbookWitchcraft",1,1,8,0,8,20
object itemstat "SKILLBOOK_UNFAIR_Rain_Fire","ea880242-8ee0-4719-b969-325b252049d3","",0,0,"SkillbookFire",1,1,8,0,8,20
object itemstat "SKILLBOOK_UNFAIR_Rain_Earth","5f972dff-9798-4474-bc47-4cc01d4859e8","",0,0,"SkillbookEarth",1,1,8,0,8,20
object itemstat "SKILLBOOK_UNFAIR_Rain_Water","ebb9d4dd-ca82-437d-b9a2-f16cb1bd9f1e","",0,0,"SkillbookWater",1,1,8,0,8,20
object itemstat "SKILLBOOK_UNFAIR_Rain_Air","361d6ffc-6099-47a9-b666-1917cf25967c","",0,0,"SkillbookAir",1,1,8,0,8,20


Now I don't know all of the variables that go into those lines, but I know the key ones.

Code
object itemstat "SKILLBOOK_UNFAIR_Rain_Air"

This line says that the object is 1) an item, and 2) what it's name within the Stats/Data folder is (SKILLBOOK_UNFAIR_Rain_Air was mine)

Code
"361d6ffc-6099-47a9-b666-1917cf25967c"

This is the Item RootTemplate that the in-game item will use. The purpose of the Links folder is to link the Stats entry to the RootTemplate.

Code
,"",0,0,

I honestly don't know what these are for.

Code
"SkillbookAir"

This one is important for getting your item to appear in-game. This is it's "object category", and can technically be whatever you want. The important thing is that this is what is looked for by TreasureTable.txt when it's looking to fill inventories.

For example, because of the "SkillbookAir" category, anything within the game that wants "SkillbookAir" within its inventory (like the Air skillbook sellers) will randomly choose items with this category. Because of this line, my book can be sold at the Air book vendors, and can be found anywhere Air skillbooks can be found.

Code
1,1,8,0,8,20


MinAmount | Max Amount | Priority | ??? | Min Level | Max Level |

The first two state how many can be stacked in your inventory I believe. I'm not sure about Priority (its 8 or 10 for skillbooks apparently). The Min Level and Max Level describe at what levels your item can be found. In my case, you can find the book starting at level 8. If you set this to 1, you would be able to purchase it from a vendor at the beginning of the game.