I have a great interest in the area of string theory, but since I am more focused on mathematics, I was wondering if there is any book out there that covers mathematical aspects of string theory. I did some research and found some resources, at these notes: http://www.mathematik.uni-bielefeld.de/~rehmann/ECM/cdrom/3ecm/pdfs/pant3/dijkgr.pdf 
and at the website 
http://superstringtheory.com/math/math2.html. Is there any book that covers string theory in more of a mathematical aspect?