Unlike most other services, Google does not have a specific device it wants you to buy in order to read its books — there's no Google-equivalent Kindle, Nook or Sony Reader. Instead, Google's trying to take advantage of the devices you already own. You can read the books online, through your Web browser, while keeping your books stored "in the cloud." (The plus side of this is you'll never have to worry about deleting your books. The downside is you need an Internet connection at all times while reading.) If you already own one of the aforementioned ebook devices (or an iProduct), things are a little bit more simple: Free apps will allow you to "cache" the book for offline reading.
Google's also allowing indy bookstores to get some ebook action by rebranding the store. That means if you wanted to support Powell's (which you should!), you can buy a book from "Powell's eBookstore" and still have it show up on your Google bookshelf.
It's unclear at this point what Google's entry means for the ebook market, or even how large the library is. They seem to have the most popular bestsellers available, but checking my three most recent purchases from the Kindle Store (Boardwalk Empire, A Supposedly Fun Thing I'll Never Do Again and The Simpsons: An Uncensored, Unauthorized History), only two were available, and for the same price as Amazon.
Nevertheless, it's always good to see new things in the ebooksphere, unless you're one of those people who insist on fetishizing the smell of paper. And if you are one of those people, I suggest you buy this ebook about the history of the (physical) phone book and watch your brain melt out of your ears.