If more books were released, I'd buy them in a heartbeat, even if only to read the margin notes between Harry, Bob and Will.
I don't think we really need any more books, though. One of my favourite things about the books as they are is how much Will says the write-ups are subject to speculation, so groups can add, remove or change what they want. Also, Small Favour ends with the perfect balance of power to start your own campaign, in terms of both PC and antagonist options. Adding more books as the series goes on might risk creating too much of a Dresdenverse metaplot.