[MaysonL]

Robert Heinlein's "On the Writing of Speculative Fiction" ranks pretty highly up there. Starts off with the old Kipling quote:

  There are nine and sixty ways
  Of constructing tribal lays
  And every single one of them is right!

and ends up with his rules (which he describes as "business habit")

1. You must write.

2. You must finish what you write.

3. You must refrain from rewriting except to editorial order.

4. You must put it on the market.

5. You must keep it on the market until it sells.

I refrain from posting the Google Books link to the essay: it's much too long, and so is left as an exercise for the reader.