I'm currently trying to write a book on the origin of religion, and as usual find myself wondering how far, if at all, it's legitimate to use contractions (didn't, won't). For relatively informal writing like this there's no problem - not using contractions would seem pompous. For books, I'm not sure, so I tend to oscillate. I find there is wide variablity in the practice of writers I admire, though there seems to be an increasing trend towards informality. Just one of the dilemmas that plague the writer.