Legal Question in Intellectual Property in California

This is a question of contract, not copyright. You need to look at your agreement with your publisher.

Mr. Selik is correct. Your publishing agreement will state whether you've assigned your copyrights in the book to the publisher, or whether you've retained all or only certain of those rights. If you assigned all of your rights in the book to the publisher, you may not self-publish any derivative versions or serializations of the original book. If you do so, you are opening yourself up to a breach of contract and copyright infringement lawsuit.

Your agreement will control as to what rights you have at this time. If you assigned all book rights to the publisher, then you would be in breach of contract and infringing the copyright if you publish on your own. However, there may be a clause that says after a certain period of time rights revert back to you if the publisher is not using them.

Because writers agreements can be complex and confusing, you should have a lawyer review your contract before you take any further steps. Even if rights have not reverted to you, it may be possible to negotiate with the publisher to have it return rights to you if the publisher has no intention of printing the book again.