Besides transferring electron polarization to the nuclei, spin lattice relaxation is a very important process for DNP. Many papers and books have been published about this topic, but I recently came across a very nice paper by Maurice Goldman that describes the different formal descriptions of spin lattice relaxation process in glorious details. If you are a theory junky, this article is for you, with very detailed mathematical description of the processes.
Goldman, M., Formal Theory of Spin–Lattice Relaxation. J. Magn. Reson., 2001. 149(2): p. 160-187.