Really rather good, although - as with so many books of this type - its target audience is a little ... vague.
In terms of accessibility for a general reader, Garber gives us a neat precis of Shakespeare's life and times, followed by analyses of all the plays in the canon. No play misses out, and all are treated fairly. At the same time, this is not an "introduction to Shakespeare", no matter what the blurb may try to sell you. All of the chapters assume at least some familiarity with the play in question, or are obscure enough about plot that you'd need to have some detail to begin with. This is not an account of the play's sources, history, or fate on the stage and screen; it's a popular academic treatise. With that said, if you're building up an amateur's Shakespeare library, this is an interesting read. What may be frustrating is an inevitability: there is so much to talk about with each play that, like most books of "essays", Garber tends to pick a few points about each play and then discuss them. This is not anything like a comprehensive overview (after all, most chapters are about 30 pages), but it tackles some of the key questions academics and directors ask about each work.
For the academic reader, I'm not sure how I feel. It seems as if Garber got the commission for the book by promising a general introduction, but she can't quite keep her intelligence at bay. And, hey, I'm not complaining; her insights are valid and well-written. Unlike most Shakespeare writers, I almost never feel as if she's wandering down a rabbit-hole of philosophical ramblings. No, Garber's analyses are - although decidedly deskbound - certainly drawn from real examination of the plays in the context of William Shakespeare's time. There are a few niggles depending on your taste (for me, I dislike that old-school scholar thing of describing a character using dashes, e.g. "Lear is her father-king"), but each to their own.
The challenge is that I'm not sure if the book unites the two worlds very well. Some of the chapters are quite high-minded, and reveal little to the general reader about the play. At the same time, there were very few surprises in the book for me (and thus, I'd assume, even fewer for the full-time Shakespeare academic). It doesn't seem as if Garber is really adding to the hefty discussion on the Bard, but nor is she a Richard Dawkins, able to illuminate a fascinating-but-niche world for the general public.
I should note this is a positive review, indeed a 5-star review (well, 4.6) - in part because I admire Garber's writing, her intelligence, and her views, and in part because as a Shakespeare lover, I was engaged on every single damn page. I heartily recommend this book to people in an "in-between" stage of Shakespeare scholarship, but I'd champion the great populists like Stephen Greenblatt and Stanley Wells for those looking to get their head around the plays in an intellectual-but-understandable way.