The right thread for me. I always have some grammar questions

And here's the first one:
I've been wondering about some contractions. The book I'm using to look up things I'm uncertain about is called
Practical English Usage and there's a list of contractions inside. I'm wondering about the contraction could've and should've which can't be found in the list. I'm pretty sure the latter isn't a commonly accepted contraction. Actually even my Firefox spell checker marks it as wrong. What about the former though? Is it not so common in British English, is it not common at all (though I'm sure I've heard it more than once) or is it simply missing?
Btw, my book suggests single quotation marks for words we want to give special meanings to. The author is British though
