
Design Elements: A Graphic Style Manual (http://www.amazon.com/Design-Elements-Graphic-Style-Manual/d...)
and
The other is the complementary case studies book "Design Evolution" (http://www.amazon.com/Design-Evolution-Handbook-Principles-C...)
Also Graphically Speaking is useful if you want to know how a particular style is constructed (artistic, corporate, vintage etc.)and you need to be able to effectively communicate with other designers or clients. (http://www.amazon.com/Graphically-Speaking-Achieving-Designe...)
With respect to the topic.. good design studios get involved in a project from day one and have a tremendous impact on the outcome of the final project. It's not simply a "skin" over a technology. It's should be a critical part of your operation.
I find it bizarre that many people assume anything "artistic" requires innate talent and you either have it or you don't. Design requires constant practice and dedication to be good and ten years (or more) of experience to be truly great, just like any other discipline.
My designs from 15 years ago are laughably bad but now I can whip up some decent fairly quickly. On the other hand... I haven't programmed anything in 10 years and have a long learning curve ahead of me.
I'd recommend looking for some good designers on one of the freelance sites.. there's no shortage of choice. If you decide to go it alone, or even if you don't, I recommend picking up the book "Design Elements" by Timothy Samara. It's the best, most concise, book on the general principles of design I have yet to find. It will give you a good overview and some talking points with any designers you hire.
Good luck!