Hi Ethan,
I don't know about an official handbook, but here's the general lowdown:
The advantage of using multigrade paper with filters is that you can use one box of paper to make prints of various contrast grades. Prior to multigrade paper, a printer had "graded" paper to manage contrast, so he or she would need multiple boxes of paper on hand.
Grade 2 is indeed a good starting point, because this is the grade considered to be "normal" contrast. However, as we know our negatives are not often perfectly exposed and the prints need contrast help, or sometimes we wish to boost (or even cut down on) contrast for aesthetic/creative reasons.
So what you have are a set of filters, probably from grade 00 on up to 5 in half-steps. My method was always to start at 2, evaluate my test print(s), and change if needed. You want to do this early on, since changing to a higher grade filter will lengthen your exposure time the higher you go in grades (due to the increased density of the filters themselves I believe.)
As far as how it all actually physically works in terms of what the light is doing is somewhat beyond me so I'll leave that to the techincal photographers, but in practice, you just need to know that 2 is normal, anything lower than that will decrease contrast from normal, and anything higher than that will increase contrast from normal.
A little good tip when using the higher grade filters is to focus your negative prior to putting the filter in - it makes focusing a lot easier.
Hope that helps - have a good one!
Matt
