Much as the [b:The Rough Guide to Anime 1|6444768|The Rough Guide to Anime 1|Simon Richmond|/assets/nocover/60x80.png|6634821] was an excellent primer on anime as a medium, so the Rough Guide to Manga is to manga. The book does an excellent job giving a brief history of manga, both in Japan and in the US, and giving a good rundown of useful terminology for the beginner before getting into the "Manga Canon".
Further, the 50 volumes in the Manga Canon are books that I can't really argue with, covering books both recent and past, and books which I'd consider significant to the medium, whether through their influence on other works, or due to their popularity. If I was to give one criticism to the canon, in the Rough Guide to Anime there were more than a few works of anime that I had never heard of before, that the book introduced me to, notably "Night on the Galactic Railroad". The Rough Guide to Manga doesn't have as much of that.
Were it not for the fact that Penguin Books has discontinued their Rough Guides series and is going to let the books go out of print, I'd consider this the perfect book to give to someone who was new to manga and was looking for a place to start. As it is, this book is still a good gift to give, and worth recommending.