Now that is quite a good question.

Does the data about the stock belong (totally) to the book itself? Or is the concept of STOCK a seperate item?

The data could go in a number of places, but how and when is it going to be used. The NUMBER_IN_STOCK is going to be reduced whenever a sale of that item is made, so does it belong in the SALES system? The NUMBER_IN_STOCK is going to be increased whenever a delivery of that item is received, so does it belong in the PURCHASING system?

Is stock control a seperate process / procedure, such that it needs its own processes?

You're the designer, these are design decisions. Make them.