Start by logging in with your Google account. Enter the book title on the Google Books page and press "Enter". On the results page, select the book you are looking for. A useful trick is to look for the free edition. Like its search Engine, Google Books lists almost all the editions of the searched book on the result pages, including the free ones. So before you purchase the book, you can try to find the free one.
Simply click the "Free Google eBooks" or "Full view" on the left side of the results page, to see all the results for free Google Books. Click on each one to open it. If you'd like to select that book, go to the top of the page, and click "Download". In the drop-down menu, choose PDF. A pop-up will appear reminding you to save the selected PDF eBook.