How many skills can the ninth continental bookcase put?

Only six skills can be activated in the bookcase, which can be replaced. Generally, it takes a set of skills to have a suit attribute.

Permanent skill books are purchased from ancient booksellers who appear randomly in the copies. The probability of rare level is very small.

There is also a hidden skill book that can be obtained by killing BOSS, which has a greater chance of producing rare grades of blue and light blue.