Part of what makes the books brilliant is that, while there is connectivity between some of them, they are all self contained. Pratchett managed to do this without being repetitive or assuming that the reader has read all the books before the one being read. There are times when characters refer to previous books but it never happens in a way that makes you feel like you are missing something. It is useful to read the different series in order but not necessary.
Part of what makes the books brilliant is that, while there is connectivity between some of them, they are all self contained. Pratchett managed to do this without being repetitive or assuming that the reader has read all the books before the one being read. There are times when characters refer to previous books but it never happens in a way that makes you feel like you are missing something. It is useful to read the different series in order but not necessary.