William M Goldman 
Rank One Higgs Bundles and Representations of Fundamental Groups of Riemann Surfaces [PDF ebook] 

This expository article details the theory of rank one Higgs bundles over a closed Riemann surface $X$ and their relation to representations of the fundamental group of $X$. The authors construct an equivalence between the deformation theories of flat connections and Higgs pairs. This provides an identification of moduli spaces arising in different contexts. The moduli spaces are real Lie groups. From each context arises a complex structure, and the different complex structures define a hyperkaehler structure. The twistor space, real forms, and various group actions are computed explicitly in terms of the Jacobian of $X$. The authors describe the moduli spaces and their geometry in terms of the Riemann period matrix of $X$. This is the simplest case of the theory developed by Hitchin, Simpson and others. The authors emphasize its formal aspects that generalize to higher rank Higgs bundles over higher dimensional Kaehler manifolds.