What are the benefits of transformational over transactional leadership?
Here is one meta analytic comparison:

http://www.ncbi.nlm.nih.gov/pubmed/15506858

Bernard M. Bass is one of the most cited researchers on this field:
Here is one of his articles but there are plenty on the net:

http://www.apa.org/pubs/journals/releases/apl-882207.pdf

I think he is also author of MLQ - multifaceted survey of leadership.

There is also revised theory by Bass and Avolio:
The implications of transactional and transformational leadership.

And also: Re-examining the components of transformational and transactional leadership using the Multi-factor Leadership; Journal of Occupational and organizational psychology. 2010.