The field of terahertz physics does not boast many elementary books. (I hope you have enjoyed this one.) Why not pursue the subject further through some of the recent books that take the subject to a higher level?

Sensing with Terahertz Radiation

Daniel Mittleman (Editor)

(Springer, 2003)

An edited collection of articles focussing on the detection of terahertz radiation.

Terahertz Optoelectronics

Kiyomi Sakai (Editor)

(Springer, 2005)

A book with much detailed technical information, featuring Japanese contributions to the field.

Terahertz Spectroscopy: Principles and Applications

Susan L. Dexheimer (Editor)

(CRC Press, 2008)

A book with much technical information, especially concerning spectroscopy.

Intense Terahertz Excitation of Semiconductors

S. D. Ganichev and W. Prettl

(Oxford University Press, 2006)

The subject matter is limited in both method (high intensity radiation) and the materials under investigation (semiconductors), but the treatment is definitive, even magisterial.

Principles of Terahertz Science and Technology

Yun-Shik Lee

(Springer, 2009)

A comprehensive overview citing much recent research literature.

Introduction to THz Wave Photonics

X.-C. Zhang and Jingzhou Xu

(Springer, 2010)

A volume summing up the recent research and with a strong emphasis on imaging.

Terahertz Techniques

Erik Brundermann, Heinz-Wilhelm Hübers and Maurice FitzGerald Kimmitt

(Springer, 2012)

A modernisation and amplification of Kimmitt's 1970 classic, ‘Far-Infrared Techniques’.