I already got a reference stating the values of most solvents, but not including glycerol and N-methylformamide. I'd appreciate if you could help in this matter.
I'm not sure whether this is what you mean, but here's paper that classifies a variety of solvents according to two parameters that --as far as I understand-- largely reflect polarity and polarizability. Maybe it can help you.
R. Carlsson, T. Lundstedt, C. Albano, Acta Chem. Scand., Ser. B, 1985, 39, 79–91.
Sure. The paper is entitled "Screening of Suitable Solvents in Organic Synthesis. Strategies for Solvent Selection." The DOI number is: 10.3891/acta.chem.scand.39b-0079. Here's a link to the journal issue where the paper is in: http://actachemscand.dk/volume.php?select1=5&vol=391
Although that paper is interesting and does list many liquids. Glycerin (glycerol) does not appear in the list. So this appears to be an open question yet. If someone finds the actual value, I too would like to see it.