Kaplan's GMAT math prep book has a solid coordinate geometry section. Kaplan's GMAT blog also addresses coordinate geometry. There are only a handful of principles that are regularly tested on the GMAT, most of which revolve around y=mx+b and the distance and mid-point formulas. If you thoroughly understand the slope intercept equation you should be able to master most anything that GMAT throws at you. The test seldom goes beyond basics. They do make things difficult by approaching problems from a different angle or asking questions in a different way than you are used to. Practice taking the question and the information provided apart and comparing the result to the basic coordinate geometry tools to see which are useful.
Tani Wolff
