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?
I would recommend S.T. Yau's book on Mathematical Aspects of String Theory, following @Tomas Smith. There is also a two volume set based on lectures given at Princeton. The books can be found on Amazon at http://www.amazon.com/Quantum-Fields-Strings-Course-Mathematicians/dp/0821820125 and http://www.amazon.com/Quantum-Fields-Strings-Course-Mathematicians/dp/0821820133/ref=pd_bxgy_14_text_y. Then, there is of course the "Big Yellow Book" on Mirror Symmetry by Vafa, Hori et.al This is available in a pdf form from the Clay Maths Institute: http://www.claymath.org/library/monographs/cmim01c.pdf
You can also find the notes on the course website: https://www.math.ias.edu/qft
Of course, there are more mathematical reviews on string theory which can be found on the arXiv. An example here is a set of notes by Brian Greene on String Theory on CY manifolds http://arxiv.org/abs/hep-th/9702155. You can also find numerous notes on string theory and connections to number theory, geometry and algebraic geometry online.
Hope this helps.