Reference request: superconformal algebras and representations

I am looking for a book/monograph which deals with superconformal (vertex operator) algebras and their representation theory. What are some good books to understand to begin with the definition of a vertex operator superalgebra and then go to superconformal algebras? I am especially interested in the $N=1$ and $N=2$ algebras (as called in physics), but all the papers and books I encounter seem to be written by physicists. Is there any mathematical literature in this direction?