ePub books will try to open with the ADE only.
But if your book is of "pdf" format. It will try to open with "Adobe Reader/Acrobat" by default.
You can reset it by:
Right click on the book.
Go to properties.
In the general tab, you will find "open with".
Click on the "change" button and set it to the ADE.
Click on the "Apply" button and ADE is set as default ebook reader.
Side effect of this is that the normal pdf iwll "by default" open with ADE as well.