Here are a few essays I found interesting. I'll add more links as I discover more good stuff.
- Andre Norton on Her Writing
- Fantasy Heartbreakers
- Heinlein's Rules for Writing
- Hollow Victories: Warlord's Burden by Trey Causey
- How to Create a Religion in your Spare Time for Fun and Profit by M.A.R. Barker
- How to Write a Novel in 3 Days by Michael Moorcock
- How to Write Suckitudinous Fiction by Holly Lisle
- Lester Dent's Master Fiction Plot
- On Thud and Blunder by Poul Anderson
- Plausibility in Fiction by Ursula K. LeGuin
- The Simple Art of Murder by Raymond Chandler
- Thank You D&D by Bill Ward
- Tom Swift: An Appreciation by Jeff Duntemann
- Whatever Happened to the Omnicompetent Man? by C.C. Finlay