Where is a good source to find which plants should be left alone, cut back, seeds gathered, etc. in the winter?
Not sure if your referring to mostly herbaceous or woody plants, but an absolute must source for any gardener is Manual of Woody Landscape Plants by Michael Dirr, which discusses shrubs and trees in depth including propagation and some fun "author humor".
For herbaceous plant care, I generally cut all perennial plants back in fall, especially if they have diseased foliage (which should never be added to the compost heap: straight to the trash with that!) and composting the healthy cuttings. Annuals, in my opinion, are best simply purchased, partly because so many of those annuals are patented so who knows if the seed is even viable, and also unless you have a greenhouse or time to create greenhouse conditions, will this plant even grow to its full potential? Biennials seed themselves and the seeds actually need to go through the cold winter to germinate, so gather only if you want to give seed to a friend or experiment with.
So I think your best bet would be to grab a thorough (not a "coffeetable" book!) perennial, biennial and annual book, Allan Armitage is an excellent author to look for.
And a propagation book that gives species-specific info is always handy.