Regularity of p-Laplacian: C^{1,\alpha} regularity and Uhlenbeck
structure.
Suggested books and references:
Giaquinta, M., and Martinazzi, L., An introduction to the regularity theory for elliptic systems, harmonic maps and minimal graphs, second ed., vol. 11 of Appunti. Scuola Normale Superiore di Pisa (Nuova Serie) [Lecture Notes. Scuola Normale Superiore di Pisa (New Series)].Edizioni della Normale, Pisa, 2012..
Giusti, E., Direct methods in the calculus of variations, World Scientific Publishing Co. Inc. River Edge, NJ, 2003..