Breach of contract question

I am a published author who signed a 15 book contract with a publisher March 1, 2016. Upon signing that contract, I was promised a $15,000 bonus to be split up five ways...3,000 upon signing, 3,000 after I turned in the first four books, 3,000 after the eighth book, 3,000 after the twelfth book, and the final payment after submission of the last three books. I submitted all books and all 15 were published on Amazon.com. On September 26, 2016 the publisher contacted me stating that he heard I was speaking bad about him and whatnot, so he wanted to end the contract. At that point, I had only two more books left of the 15 books. He released those books in October, after he said the contract was null and void. If the contract was null and void, then he should not have released the last two books, but he did and now he won't pay me the last 3,000 that I am owed. Do I have a case from a legal stand point b/c in my eyes, I fulfilled my end. BTW I live in Texas but the contract is under GA law