Its tough to say without more detail. But this is an interesting topic, because situationally, in a scenario like this, patents, trademarks, and copyrights can all enter the equation at the same time.
If your product line primarily consists of, well, art...yes the art will be covered by copyright law and protection will be automatic. However, if your product line is, say, a decoratively shaped and/or designed dinner plate w/decoratively shaped accessories - which are all meant to be used and not to simply sit in a musuem and look pretty...it's possible that copyright law will not come into play, there is gray area. Design patents are meant to protect the ornamental, non-functional design of an otherwise functional product. Under US law, there is generally not supposed to be overlap between a design patent and a copyright. Of course, this is where complications can arise, because if you tried putting a famous, copyrighted painting on a t-shirt, I am fairly sure you still run afoul of copyright law (i.e. where confusion exists and stakes are high enough, you could do a bit worse than consulting with an attorney).
If what you have falls under the category of "design patent," the good news is that filing a good design patent is quite a bit cheaper than filing a good utility patent. The bad news is that you will only be offered relatively narrow copyright/trademark-esque protection.
You may also, over time, be able to develop trade-dress protection, a type of trademark protection. i.e. if you got to a point where relevant consumers clearly associated particular product designs with you or your company, you could claim trade dress protection, which would apply automatically (although registration would have benefits). This protection would never expire until you stopped marketing your product line, too. In a sense, you can theoretically use a design patent to help buy yourself the time needed to obtain trade-dress protection. Anyways, for more information, Google "trade dress."
If you are going the design patent route, NDAs before you have filed are probably smart. Should you have a notary sign an NDA? I haven't heard that question before, and it's a good one. I'd imagine that would be smart. I am unsure if notaries have a code of ethics to follow and whether confidentiality is implied.
I would say, you might as well get the notebook notarized. But, be aware of the limitations of an inventor's notebook - http://theinventorsmentors.forumo.biz/is-this-true-f28/inventor-s-notebooks-t253.htm.
Meanwhile, if you believe that what you have is protectable by Copyright law - register your copyright(s). Registration is relatively cheap and beats any random notebook re:documentation (at least for copyright-able stuff). Protection is automatic and does not require this, but again, registration is relatively cheap, and it will make it easier for you to enforce your copyright protection down the line if need be. Learn more at Copyright.gov...