Is it permissible to take books from my current workplace when I leave, knowing that some of them are not used by the institution?

If the books have been given into your full possession, there is no objection to keeping them and disposing of them as you wish. However, if they were lent to you for use in your work, then you must return them to the responsible authority when you no longer need them, and it is not permissible to dispose of them. You can ascertain the nature of how the books were given to you by referring to the responsible authority.