Mac: How to make Bookshelf your default EPUB reader
First, open the Finder app from your dock and locate an EPUB book. Right click on the EPUB book and click on Get Info.
This will open up details about the book. Find the section labeled Open With. You may need to click the arrow next to the words "Open With" to display this section. Next, click on whatever app is selected to see a list of apps to choose from. Click on the Bookshelf app.
Next, click on the Change All button to make all EPUB files open with Bookshelf.
Finally, click Continue to complete this change.