In each book, Paizo publishes the Open Gaming License, usually in the back of their books. Also, they publish what is their Product Identity and what is open content, usually in the front of their books.
Study the OGL and look at multiple books by Paizo... After all they are using material from the third edition of D&D themselves and have to follow the rules of the Open Gaming License. If you use anything that is open I think you need to cite the reference in your book's OGL declaration. Note: Almost no pictures, names, dieties, stories, locations, trade dress, are open unless something previously declared those open.
Also, at PaizoCon I believe Paizo offers classes for people who want to make third party products. That might help you understand what is allowed.
Look at other third party books and see how they use the OGL for ideas.
If you want to be really safe... Hire a lawyer and get his help with understanding what to you can and can't do with the Open Gaming License. I am not a lawyer and never want to develop my own material to sell so my advice should be considered worth what you paid for it.