If you have two types of one flower (ie, different colors of cosmos, different varieties of the same flower like your amaranthus) I am pretty positive that they will cross-pollinate. Love lies bleeding in itself will probably come true from seed as long as it has not been cross pollinated. I got some that had been collected from someone elses garden, and it did come true this year. Don't hesitate to plant what you've got--you can get some interesting things sometimes!! (And alot of seedlings, even when cross pollinated, will mirror the parent plant fairly closely!)

You can get a general botany book at a bookstore or college bookstore. There was a book out years ago called Botany for Gardeners.

In addition to what Hollywog said, if you keep crossing hybrid plants, you will wind up with the original non-hybrid strains, which are sometimes beautiful, but sometimes may be not worth the garden space. You may also come across a plant that is so unusual, like my 6 foot celosias. That's part of the fun of gardening.

Bummer, does this mean I can't trade my Amaranth seeds that I've collected? I had several varieties that I had collected seeds from and was looking forward to having something to trade with (I'm new to collecting seeds this year). I had a maroon Celosia that was about 7 feet tall too that I collected seeds from. Also, it sounds like even though I saved seeds from the yellow marigolds they may turn out to be yellow or orange, right?
I had checked out a book on Botany once from the library and it was all over my head. It was really technical stuff. Are there certain plants that I don't have to worry about cross-pollination with? Thanks for all your help! :)

For trading, just make sure you are clear about them being "open pollinated". Most traded seeds are.

Just off the top of my head here...
Columbines- Hybrids will cross pollenate with other hybrids in the immediate area, and may revert to orange. However, if they are not near other varieties when flowering, there s a good chance they will come true. (ie., I planted 'William Guiness' from traded, OP seeds, came true. I'm still waiting for my other columbines to bloom, and there are no others in the area, so I assume the seeds I collected will come true, or at least very close to the parent plant.)
Petunias- I grow several varieties each year, and collect seed to winter sow. Every once in a while, I get a white one. I rarely grow white, its usually purples and blues, so I assume these were cross pollenated.
Snapdragons- They are self fertile, and most will come true. However, if you want to try it, they will cross pollenate. You have to pinch open the flowers as soon as they bloom, and pollenate with another species. If you try this, you will probably get mixed seed from the pod, some in the form of the parent plant and some that have cross pollenated. (Snaps are "born pregnant!")
Cannas- Hybrid cannas rarely come true from seed, regardless of what else is blooming nearby. I have read that most revert to red.
Morning glories- Hybrid varieties of i. purpurea (grandpa otts, kniola's black knight, milky way, "old fashioned mix," star of yelta, etc.) will come true for a few years, but will revert to purple eventually, especially if planted with others of the same species. I am not sure about i. tricolor (tie dye, flying saucers, heavenly blue, etc.)

Garden Witch,

I was sure that Grandpa Otts was a Heirloom variety an original from the Bavarian varieties as it is offered by the Seed Savers Exchange.