To search for online books in Google Books:
Enter in a keyword and search. Select Free Google ebooks from the left coulmn to view fulltext books that are available for free online.
Select Preview Available to view excerpts of books available for free on the internet.
To search, type in your keyword (topic) into the search box and select Search. Once results are retreived, click on a title that you would like to view:
After selecting the title, click on the eBook Fulltext Link to access and read the book online: