First of all, here is our Help mepage for database assistance.
Secondly, I suggest that you search a couple of databases: Academic Search Premier and Business Search Premier with these terms:
leader, leaders, leadership, and effectiveness.
Hopefully, these terms will return good articles for you.
As to a book suggestion, if you search our database Ebsco eBooks using the terms leader, leaders, or leadership, a list of downloadable eBooks will return for you.