See this sample. Can't be done in normal ebook formats. And the scan is quite high rez, as I can see if I view it on my Mac in a web browser. I can zoom in or out. And I can search the book. Excellent.
Unfortunately and weirdly, these things are not true in the iPad app. You can't zoom in, and you can't search. Both are highly unfortunate omissions for the most attractive reading platform we have for illustrated books, so I hope they get corrected very soon.
I must say, apart from the great number of out-of-copyright titles of course, the Google ebook store is not exactly cheap. I have not done thorough tests, but in general browsing, the titles I see range from ten dollars to fifty dollars. No, wait, it's Pounds Sterling, all the worse. I do see much lower prices occasionally, but my impression is that there is very far from the emphasis on affordability which you find on the Kindle book store.
In any case, choice is great, and it's lovely that at least one bookstore has the graphic-heavy books at this early point in ebook history.
would you care to, briefly, explain the need for it to be in PDF format? For instance: what is it that can't be done in a normal e-book format?
The common ebook formats like Kindle (mobi) and ePub are based on web code, HTML, and like HTML they are designed to be able to be shown on screens of varying sizes. Which means that the text can re-flow to a smaller or a wider screen, and this means that the design has be be kept simple, and those limitations are built into the format. Also it must be shown by very low-powered devices and transferred on sometimes slow Net connections, so that means it is basically just text, with perhaps the occasional small picture thrown in between paragraphs.
PDF on the other hand is originally mainly designed for print, so that means that almost no matter how fancy your layout gets, pictures and text on top of other pictures, complex tables, etc, the page looks exactly the same on any screen or any printer. It's actually an amazing format.
The downside is that often the files are much bigger, and a small screen obviously does not show a big complex page very well. So publishing an ebook on PDF is not easy, you have to find some kind of compromise between 4-inch screens and 30-inch screens, and print! That's why the other formats are preferred unless you have to have lots of graphics or complex tables or such.