I have both. And I don't think there is ever any good general answer to such a question, so...
It all boils down to your own preferences, interests and styles. If you are serious about Copperplate and only Copperplate, I'd be tempted to say "get the first" (Mastering Copperplate Calligraphy), but if you are curious and just want to learn the basics and are also curious about italic, then second book (Italic and Copperplate) would be better.
I find the first ("thinner") book, thicker to read, but it gives a lot more detail on the basics and works them in minute detail, which is OK for those who like to reinforce their learning and encompass minute aspects of a subject. The second gives the basics and a lot of practical, illustrative ideas, it is more pedagogic and lighter to read. Kind of comparing a French encyclopedic textbook and an American illustrated text. It all depends on the kind of learner you are.
If you want Calligraphy, the first is better, if you prefer to improve everyday writing I'd say the second.
Some people delight in minute detail, some only want some reference examples and prefer to work their own way out.
"Italic and Copperplate" is intended to give you fast results, "Mastering Copperplate" to guide you slowly to perfection.
And I could go on for hours. Briefly: as a beginner I'd probably start with the easier, less technical, more appealing "italic and Copperplate", but keeping in mind that if I intend to seriously pursue Copperplate, I'll proceed to "Mastering Copperplate" afterwards. Thus if you seriously want to "go for it", and don't mind the learning curve, maybe it is better to directly start at the definitive reference.
If you are the kind who can work his way out and prefer to figure things by yourself, and would rather have only a quick reference, there are other booklets that may serve you better.