I have written a number of articles for books or conference proceedings which are somewhat less technical than regular journal articles. A partial list is below:
Quantum gravity via supersymmetry and holography (with H. Elvang) in General Relativity and Gravitation: A Centennial Perspective (eds. Ashtekar, Berger, Isenberg, and MacCallum) Cambridge University Press (2015).
Introduction to holographic superconductors in From Gravity to Thermal Gauge Theories: The AdS/CFT Correspondence (ed. E. Papantonopoulos) Springer-Verlag (2011).
Gauge/gravity duality (with. J Polchinski) in Approaches to Quantum Gravity (ed. D. Oriti) Cambridge University Press (2009).
Spacetime in string theory New J. Physics 7 (2005) 201.
Playing with black strings in The Future of Theoretical Physics and Cosmology (eds. Gibbons, Shellard, and Rankin) Cambridge University Press (2003).
Quantum gravity at the turn of the millennium in Proceedings of the Ninth Marcel Grossman Meeting in General Relativity (eds. Gurzadyan, R.T. Jantzen and R. Ruffini) World Scientific (2002).
Black holes (with. S. Teukolsky) in More Things in Heaven and Earth: A Celebration of Physics at the Millennium (ed. B. Bederson) Springer (1999).
Quantum states of black holes in Black Holes and Relativistic Stars (ed. R. Wald) University of Chicago Press (1998).