My understanding (from the mini encyclopedia of Guinea Pigs mostly) is that mown grass should not be given fresh, as it heats up (and starts to ferment) quickly, which I guess could cause bloat or other tummy problems. I think the book is assuming the reader is considering feeding mown grass from a domestic lawn, but I would think that long grass from a field would be the same. I wouldn't risk giving it while green, but it may be ok once it has dried and turned to hay - I'm not an expert though.
I hand pick grass from my lawn for my pigs, and occasionally when it is long and the weather is sunny, I spread some out to dry for feeding when I don't have time to pick grass.
As to plants they shouldn't eat, the book lists among others buttercup, bindweed, daisy, dock, and scarlet pimpernel, which might be found in a grass field.
Hope this helps!
