The Complete Gettysburg Guide for the iPad Kindle is slick and well worth the $13.77 price tag. It is in full color and easily navigable like other books. In addition, you can zoom in on the maps.
To demonstrate some of the functionality, here is a video demonstration of the book.
I have used the hardcover edition for nearly two years now and it is my favorite guide. The Kindle edition just made it much easier to carry this thing around. If go to Gettysburg regularly, this book is invaluable. Even if you go once, this will serve as a great guide to help you find the areas of the field that are important to you.
If you do not have an iPad, the book is also available in hardcover.