A modern classic, collecting together essays by one of the most prominent recent Bach Scholars. Several essays are of direct relevance to the cantatas. I found both this and Marshall's book extremely useful in bringing me up to date on modern thought about Bach's work, especially in explaining the context within which the cantatas were written and by explaining the ambiguities in the original sources that lead to interpretative difficulties. Robert Marshall , The Music of Johann Sebastian Bach , Schirmer, 1989. Another modern classic, collecting together essays by another of the most prominent recent Bach Scholars.
Several essays are of direct relevance to the cantatas.
See the entry for Wolff's essay collection. An excellent and valuable source of current thinking about the context and background of Bach's works. Christoph Wolff has done us a great service in updating David and Mendel's classic book. English translations of the most important of the documents relating to Bach's life and works. The standard catalogue of Bach's works.