My motto when it comes to essays about writing is, "Take what's useful and discard the rest." Just because an essay says you shouldn't do something doesn't mean you shouldn't. Just that you should try to do it really well. :D
How to Write a Crossover Fanfiction Story
Tips for Writing a Crossover Fanfiction Story
This is more of a "history of crossover fiction" type deal. But interesting.
Thoughts About Crossovers In General
Very useful essay. Especially the tips on devices you can use to bring fandoms together in your story.
A Brief Guide to the Writing of Crossovers
Bridging Universes: How To Write Crossover Fiction
The Ten Commandments of Crossovers
I actually disagree with a few of the "commandments." But some should definately be kept in mind.