Sign into Google with your Google account, go to Google Books and you'll see a menu on the left called 'My library'. When you find a book you want to save, just click on the 'Add to My Library' link in the left-hand menu. If you've created new bookshelves (ie. categories) you can add the books straight into them. I've so far categorized my bookshelves by surname.
Once a book is in your library you can add a note. This is where I write a brief explanation of what I've found of interest in the book together with the page number so I can easily identify what I'm looking for when I'm ready to document my sources.
'My Library' in Google Books
[Why Sorting Saturday? This phrase has been included in the title in order to take part in Daily Blogging Prompts at Geneabloggers]