Here's a list of resources on cats and litter box problems which either support the information I have published on Cat Lovers Only, or offer additional insight or a different perspective on the problem.
When it comes to solving cat litter box problems you need all the help you can get. These websites will give you a decent range of viewpoints and techniques with which to approach the problem. Remember to consult your veterinarian and rule out any physical problems.
Cats International offers information on feline behavior including training techniques and behavior modification. Problems such as destructive scratching, inappropriate elimination or house soiling, and aggression have been successfully treated.
The Dumb Friends League is a private, nonprofit organization founded in 1910. They provide a wide variety of resources for cat lovers, including some of the best advice on cat litter box problems.
Veterinary Partner has a good page on the differences between spraying/marking territory and inappropriate elimination.