On each upgrade icon, you see a number, that number indicates which level your skil need to reack to unlock that upgrade ability.
If you see "1" upgrade in upgrade is available on first level
If you see "2" upgrade is available on second level, and you need 3 same skillbooks to unlock lv 2
If you see "3" its available on third level, and you need 6 more same skillbooks to upgrade from lv 2 to lv 3.
Hope this clarifies to you.