This application is, as its name clearly says, a program that helps you download Google books on your Mac. The utility provides you with a straightforward interface where the only thing you need to do is paste the Google book URL and the app will download.
You can use your default web browser to find the books you need to download to your Mac. In my opinion, it would be a lot easier if the program could provide you with an integrated browsing tool to immediately find and download the necessary books.
The time you spend waiting for the output files strictly depend on your Internet connection signal. Once the download process is complete, you can go to the selected destination folder and check the PNG files using Mac's default image viewer (Preview) or other third-party apps.
Sadly, the application comes with several disadvantages: it downloads only the books marked as Full View (the books are out of copyright), it saves book pages as separate files (not so convenient if the book has more than 50 pages), and it supports only one output format (PNG).
I'd say it's worth installing it on your Mac as long as the aforementioned flaws don't mean much to you and you're happy with this app's features. The app is free of charge, hence, you won't loose anything if you give it a try.