I can recommend the following two books:
Alphonse Mucha, Sarah Mucha. (2005, Francis Lincoln in association with the Mucha Foundation) ISBN 0 7112 2517 6
This book is a great way to put Mucha and his art in context. The text is informative. Overall the reproductions are good—but not always excellent (which I'd expect from the Mucha Foundation). You can usually get a sense of the metallic inks (difficult to show in reproduction) but in some images the color is not even close to the originals. The page 49 Medee poster is an example of poor color matching. What you do get in this book are relatively large reproductions—this is a large coffee table book and often images are given a good portion of the page. You also get examples of his art from across his life. This includes several pages of drawings. (I would like a book of nothing but full-sized reproductions of his drawings.)
Mucha, Renate Ulmer. (2007, Taschen)
I bought this $10 book solely because the reproduction of the Medee poster was more accurate to the original I saw in the exhibit. You can see the navy lines in the reproduction and it makes all the difference to "understanding" this piece and the color scheme—which is brilliant. (A postcard reproduction of Medee from the Mucha Foundation is embarrassingly bad!)
I haven't read any of the text of this book, but it too contains lots of info on the artist and his art. There are also three pages of head studies which are a must have. I only wish the book's dimensions were larger so these full-page images would also be larger.