'Nonlinear Noninteger Order Systems: Theory and Applications' by Paolo "Arena, R. Caponetto, M Porto, L. Fortuna, M." Porto (2001) Is a good book in this area.
Areas of applicability are: Non-inductive transmission lines, 1/f noise spectra, polarization effects in dielectrics, diffusion effects and a lot of acoustic effects. Some interesting control theory arises from the use of non-integer order control systems, the most well known is CRONE (french acronym for Command Robuste d'Ordre Non-Entier) - it provides for damping that is insensitive to the parameters of the system being controlled. There are even non linear \$PID\$ systems, of the form \$PI^\lambda D^\mu \$. Image processing with variable gamma is also a very common thing that people experience.