Magnetic monopoles: Dirac and Polyakov-t'Hooft   back  
The construction of the Dirac monopole and the Dirac quantisation 
condition [1, chapter 2], [2, chapter 1] and [3, chapter 8.1]
                                                                                
The Polyakov-t'Hooft monopole and the BPS bound on its mass [1, chapter 4], 
[2,chapter 1] and [3, chapter 8.2 -- 8.3].
                                                                                
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[1] P. Goddard, D.I. Olive, Magnetic monopoles in gauge field
    theories, Rep. Prog. Phys. 41, 1357 (1978).
[2] J.A. Harvey, Magnetic monopoles, duality, and supersymmetry,
    (hep-th/9603086).
[3] N. Manton, P. Sutcliffe, Topological solitons, Cambridge University
    Press (2004).