  The first free comprehensive textbook on quantum (and classical) field
theory. The approach is pragmatic, rather than traditional or artistic: It
includes practical techniques, such as the 1/N expansion (color ordering) and
spacecone (spinor helicity), and diverse topics, such as supersymmetry and
general relativity, as well as introductions to supergravity and strings. The
PDF version can be more convenient than paper books, with Web links and a
clickable outline (contents) window.
