Are they? Bugger, I am reading Tinker at the moment. I watched the BBC TV series with Alec Guinness back in the 90's (obviously not on it's original run), so I know the story, so I suppose I am not revealing anything new to me. But I wished I had known it was part of a larger arc, it certainly reads a bit like one.
I would read them in the order they were meant to be read, so original published date. Some "prequels" read better on you knowing the middle/end bit first.