Recently, I wrote a post about how to access Picasa Web Pictures on Galaxy Tab. Today, I will guide you access offline other Google's sevice that is Google Books. Below it's processing:

Step 1. Open the Google Books application and search Books that you want to read while offline.

Step 2. Press and hold on the book's cover. A small menu will pop up.

Step 3. Select "Make avaiable offline"

Step 4. You can add more other books by pressing the small drawing-pin under each book's cover.

Step 5. Once finished, press the "Done" button in the lower right corner.

You can see how much space you have available for downloads during the book selection. 
And it's OK to pick all titles, as long as you have the space.