Swan pisze:
"The rules about hyphens are complicated, and usage is not very clear. Perhaps because of this, people seem to be using hyphens less, especially in compound nouns. (...) The situation at present is rather confused, and it is not unusual to find the same expression spelt in three different ways (e.g. bookshop, book-shop, book shop). If one is not sure whether to use a hyphen between words or not, the best thing is to look in a dictionary, or to write the words without a hyphen."