I have to admit that I'm not ready to read everything online.
Call it a middle-age thing, but I still prefer the feel and portability of an actual book. I'm willing to be persuaded otherwise, but for now I prefer books on paper rather than on a screen.
Which is not to say that I don't find books useful. And particularly when the book is way, way out of print, Google Books is a very useful tool.
As an example, I had long wanted a copy of Charles Porterfield Krauth's The Conservative Reformation. And none were available. The book was long out of print, and when used copies would come up for sale on Amazon, they would fetch astonishing prices. Say, $150. In other words, more than I cared to spend.
Along comes Google Books. Google Books has the modest goal of printing at least a preview of every book ever printed. I can't say how far along they are; there are a lot of books that have been printed. But a book like Krauth's (now actually back in print thanks to CPH) is available, and you can actually read the entire book online if you are so inclined. The secret is that Krauth's book was published before 1922 -- which means that it -- and every book with a pre-1922 publication date -- is out of copyright, and anyone who wishes can print it.
Books go out of print for a variety of reasons. Some very fine books go out of print for long periods. What you will be surprised to find is how many useful books there are out there that are no longer being sold.
Here's how you do it. Go to "books.google.com" In the search space at the top, type in something: "Lutheran" or "theology" or "liturgy." Now look for "full view" at the top, and click on that. The next page will show books which are available for reading the entire book online. 99% of these will be out of print books.
What I find very useful with this service are finding very narrow, very specialized information. For example, if you're interested in regional church histories, there are congregational histories that were printed in very small press runs, and copies almost impossible to locate. But Google Books gives you the opportunity to read such materials, information that would have been unavailable even 20 years ago.