Hmmm... I kind of feel like a sucker. I started reading through "Good Photos in Bad Light, by Darwin Wiggett". It started out fine, but about 3 pages in, I realized that it was a long-ish advertisement for a filter manufacturer. I guess there really is no free lunch. There is some good information in there, so I'm not detracting from Francois' find, but I just feel like I should have known when I saw "Free" that there was a catch. It also seems a little silly to put filters in front of a DSLR. You should just shoot RAW and do all of your adjustment in post IMHO. Sorry to go down the d!&!74L rabbit hole there.
Well... as they say: you don't look a gift horse in the mouth...
As we all know, free does not necessarily equal quality... but sometimes it does. And when it does, it's always one of those wow moments...
BTW, love your usage of L33T