Click on Add to Firefox and then on install Now button.
Now Restart your Firefox and you will see a monkey icon adjacent to your Address bar.
After installing this you have to install a user script > Click Here
For this click on the install button and again on install button.
Now you will see a message displaying Google Book Downloader installed successfully at the right bottom of your screen.
Now go to the Google book store and find your book there.
Here you will see a button saying Download this Book in the left panel.
That's it. Now you will be able read your desired book offline.
Note:- If you are not able to download book on clicking Download this Book button then install Flash Got Addon in your browser. After installing the addon right click on the Download this Book button and select FlashGot Link option and then click start download button.