To read:
\citet{Mauerhan_2017}: SN 2013ej

Review papers: 

\citet{Wang_2008}: Spectropolarimetry of Supernovae
This is a very observation oriented review covering specpol of both Ia and CCSNe. It has an exhaustive list of all SNe with polarimetric observations, both photometric and spectroscopic. For CC, it went through all the subtypes, and conclude that all CCSNe are significantly polarized. The common II-P events have small polarization during the plateau then experience a jump in polarization as the SN enters the nebular phase. The interpretation for this observation is that the asymmetric inner core region is revealed. The best observation of this is from SN 2004dj (Leonard+ 2006). This interpretation is corroborated by the fact that IIb and Ib/c are polarized early on since the core is already revealed, although observations for these are sparse.
Morphology of the polarimetric spectra on the qu plane can be helpful. Recall that the angle of polarization is 0.5 arctan(u/q), so if a SN is axisymmetric, its specpol will fall into a line in the qu plane (constant PA, thus constant u/q). Any scatter perpendicular to that is a sign of deviation from axisymmetry. Varying PA across a line can produce a loop in the qu plane. 
\citet{Mauerhan_2015}

IIn

\citet{Reilly_2017} FORS2 Optical specpol for SN 2009ip during the 2012 outbursts. 
About SN 2009ip
Polarimetric observations by FORS2 on the VLT, 35, 42, 64, 68, 73, and 83 days after the 2012b outburst
Interpretation of polarimetry

II-P