Can my employer gain rights to my book if I sign a non-disclosure agreement?

Your treatment center employer may well be concerned about a couple of issues. First, there is an outside possibility that your employer can gain rights to your book. While generally the author is entitled to the exclusive copyright rights in her work, this could change if, for example, your employment contract or employee handbook states that any work created becomes the property of your employer. Whether or not your self-help book could reasonably fall within the scope of "any work" is certainly an open question.

The other issue concerns the center's liability. Signing a confidentiality/non-disclosure agreement may not give them enough peace of mind.

You could sign the agreement and violate it anyway. Because the center can't prevent third parties from suing, they may demand an opportunity to review your manuscript to ensure that you don't, in fact, disclose any client confidences of intellectual property of the center.

They may also want you to procure your own insurance to cover any liability for these outside business activities. In either event, speak to an attorney in your area who is familiar with publishing law and liability to make sure you steer clear of these thorny areas.