Just published in the SAGE encyclopedia of educational technology.

Koehler, M.J., & Mishra, P. (2015). TPACK (technological pedagogical content knowledge). In J. Spector (Ed.), The SAGE encyclopedia of educational technology (pp. 783-786). Thousand Oaks, CA: SAGE Publications, Inc. doi: http://dx.doi.org/10.4135/9781483346397.n318

You can read our entry on TPACK here, or get the book here.