1-4 of 4 Books

Harmonic maps, conservation laws and moving frames
By Frédéric Hélein

Asymptotic Behaviour of Tame Harmonic Bundles and an Application to Pure Twistor $D$-Modules, Part 2 (Memoirs of the American Mathematical Society) (Memoirs of the American Mathematical Society)
By Takuro Mochizuki

Harmonic maps into homogeneous spaces
By Black, Malcolm.

Harmonic maps
By National Science Foundation (U.S.)