Here are 3 papers on N-sided patches.
Looks very mathematical :-)

http://hal.inria.fr/docs/00/51/79/41/PDF/Kan-LeShi2010d_CAD.pdf

http://hal.inria.fr/docs/00/51/83/26/PDF/Yi-JunYang2006c.pdf

http://hal.inria.fr/docs/00/51/79/38/PDF/Kan-LeShi2010a_VC.pdf

The third paper is almost readable!