My friend Tim at Gothridge Manor has written a series of good posts on his editing philosophy. (Part I, Part II) His aide has been invaluable to me getting Thieves of Badabaskor, The Wild North, both Points of Lights, and parts of the Majestic Wilderlands (It was my screw up that it didn't get well-edited before it's release).
It is hard an author to surrender his work to an editor. Because a good editor will do more than just correct grammar but work with you to produce a tighter, more concise, and ultimately a better written work. What hard for an author to deal with is the change of dozens of bits of text. Many have issues even changing a few words here and there.
As an author the thing to keep in mind is the overall purpose of what you are written about. To use the example that Tim picked, Points of Light it was to present four campaign settings, each with a distinct theme. Tim challenged me at various points in my writing as to whether what I had written was extraneous wordage or truly important. Like the example of the Night Hag, he also pointed out stuff I missed and recommended that I include more.
The point of writing a game supplement isn't to hear yourself speak but rather teach another gamer what you are trying to do. Whether it is a series of interesting classes, an adventure, a setting, or terrifying new monsters nobody knows what in your head unless you write it down. Also nobody will read it unless it is well-written, clear and concise.
A good editor will help with that and Tim certainly did with my material. But you need to let go of your ego to make the relationship work.
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