I believe that Apple insisted that a number of apps selling books remove their 'buy' options - e.g. does not exist on my Kindle app - yesterday I went to Amazon via the Safari browser and purchased a book which loaded fine into my Kindle app.
The same is true w/ the Google books app - just took a look in the Apple App store and the update statement also indicates removal of the link to shop for ebooks there; via your browser, just go to ebooks.google.com.
I agree w/ you about the prices of these electronic book editions - often price is pretty much the same as the print edition - I don't understand why these digital editions should not be offered at a much reduced price, except for the greed of the publishers.
Of course, check out the 'free' book availability - some sites already mentioned; an additional one is the
Gutenberg Project - thousands of books - now these are older ones in public domain, so don't expect recent releases.
Another option is to borrow ebooks from your local library - I use an app called OverDrive - the issue there is that a limited number of e copies are available and often you need to be put on a waiting list, plus in my library (assume true elsewhere), the book is released for just 2 weeks (well 2 of mine on the list came up at the same time, so there is no way that I can complete them in that time period!).
Good luck and post back if you find some more interesting sites -