Books (advanced):

Conformal Field Theory by Di Francesco, Mathieu and Senechal

Conformal Field Theory by S. Ketov

Affine Lie Algebras and Quantum Groups by J. Fuchs

On-line resources (advanced):

Applied conformal field theory by P. Ginsparg

Conformal field theory by B. Schellekens

An Introduction to Conformal Field Theory by M. Gaberdiel