This farmer knew that the seed he purchased was subject to a plant patent; that's the reason he purchased it; so he could avoid the cost of buying it legitimately.
No there's a huge difference: plants by their nature reproduce themselves, books do not.
and, yes, the use to which the seed is put does make a difference.
Only if that use is bound (conditional) by contract... otherwise you could say that you would not sell me Diesel because I was going to use it to wash my dirty oily hands rather than burn it in my car.
This farmer knew that the seed he purchased was subject to a plant patent; that's the reason he purchased it; so he could avoid the cost of buying it legitimately.
Again, that's irrelevant -- he stole nothing, he bought the seed legally -- your view of patents would mean that someone buying a used computer that someone had not wiped with, say, a full version of PhotoShop installed is theft because it uses proprietary/patented algorithms*.
* - Algorithm patents are, by their nature, absurd -- especially with the amount of abstraction allowed in descriptions. Patenting "'f(x) = X^y + C' where y is some number and C is a constant" makes as much sense.