I have been wondering the same thing. I recently purchased a packet of 'Empress of India' nasturtium seeds from Seed Savers Exchange and it says explicitly on the packet to keep it isolated from other nasturtium varieties by 1/4 mile to prevent it from cross-pollinating. I would love to know if similar requirements exist for petunias and nicotiana.

I do know from experience that morning glories cross-pollinate extremely easily and that it is difficult to maintain pure seed.

Hi Mary,
'Seed to Seed' only covers vegetable gardens. I have not seen a book for flower seeds saving.
If you are not sure if a variety is a hybrid, a quick google search of the variety will tell you. I google the plant, the name, and seed as in "Naturtium 'Empress of India' seed". This usually leads to seed catalog listings. All good catalogs will state whether a variety is a hybrid or not.
Spacing varies between species. Some flowers can be grown fairly close while others like Sunflowers will need long distances apart. I suspect many open flowers are visited by lots of pollinators and will need long distances. Also, some flowers are self-pollinating and some flowers need pollination from another plant. (It can be very confusing to remember.)
One thing you can do for certain flowers besides planting one variety is stagger plantings or plant late and early bloomers so bloom times vary. Another thing is to hand pollinate and bag the blossoms. Or you could alternate years of planting varieties. Most seeds(there are a few exceptions) will last a few years if kept in a cool, dry, and dark location so they don't need planting every year.
If I didn't make sense or you need more info just ask : )

Ispahan,
I've seen hummingbirds go to petunias, so I assume easy crossing because of that fact. I've received crossed nicotiana seed, so I know that does too. I believe hummingbirds like it too which would explain that.

If you grow only one variety a year, and none of your close neighbors do, chances are lowered pretty well that the seed you save will be good. The 1/4 mile is too be absolutely sure, but you can do less for your own purposes. Once you know a plant, it is easy often to spot an off seedling. If you don't until it starts to bloom, just yank it then, unless of course you realize, you've got something new that you like.
Remy

What about the book "from seed to bloom" (or something like that). I am trying to find a book like the one you described. It would be nice to have a book to reference when you are learning to save seeds.