Here's a link to a site with an almost overwhelming amount of kidney info, including lots of links to kidney diet research: http://www.dogaware.com/health/kidneymedical.html#
As I recall, Batmom, whose opinion I respect, has posted that she is not impressed with the dogaware site. I have found it helpful as a starting point for my own research, but would welcome Batmom or anyone else pointing out con's to the site.
My Scout had a form of kidney disease for almost three years. He managed pretty well with a low phosphorous diet until the last 3-4 months. I gave him azodyl those last few months, which brought his kidney measures down significantly. But remember that azodyl does not improve or maintain kidney function, it apparently just helps the body break down the toxic waste products better. The dogaware site does not have a high opinion of it, and I am not a complete believer, but I would give it again.
I follow Diane Burpdog's diet advice for seniors, which is give them what they'll eat (that's not actively harmful).