The classic biography of Bach with extensive discussion of the cantatas. Way out of date in some respects, this is still essential reading. Be aware that the Dover edition is of the original English translation and the German edition contains more material. Some caution about the translation is also required. 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.