Those are both great books and great references.
Here is also a presentation that David Steinberg did for SCC. The usage trends of the preservatives is out of date since the presentation is from 2012. However these usage trends have been updated since then and even posted recently in this forum.
My advice in learning about preservatives would be first to learn the functional “groups” (for example Parabens, Formaldehyde donors, Organic acids, etc.) and some representative raw materials. This is covered well in the presentation. There is also guidance of the max levels and appropriate usage.
Once you know these functional groups I think you will see that any number of distributors be it Schulke, Ashland, Lonza, etc. will have a line of preservatives which will serve the needs of almost every Formulation. You may find that you buy 4 to 5 separate products, but these are generally available in those lines.
Look at the Cosmetic Market as well as your potential clients. In this step you will begin to see the market forces (paraben free, etc.) that will contribute to your preservative selection. You may have a demand for naturally compliant preservatives. Be pragmatic though. You do need to address your clients concerns, but NEVER let market forces influence you to create a compromised preservative system. In some Formulations for example, parabens are simply the best solution.
Next, learn and follow the general concept of the “Hurdle Technique.” Again this is covered in the link. Simply, the preservative chosen is not the sole piece of a preservative system. Other factors such as pH, chelants (that will answer some of your EDTA questions), packaging and other issues should be taken into account.
Once you have your preservative system, move on to the rest of the Formulation. Preservation is just one factor of many in creating a successful formulation. Don’t concentrate on the trees and miss seeing the forest is a great way to prioritize your efforts.
Lastly, once you have a final Formulation, don’t hesitate to test your preservative system with outside challenge testing.
We have an obligation to create a safe product. This can be done with the raw materials available and while meeting marketing demands without cutting corners. Don’t feel free to compromise on this safety.