If you have the latest version of Google Chrome, you won't be able to download any books that have Digital Rights Management (DRM).
In the meanwhile, you can use these browsers to download DRM files:
What is Digital Rights Management (DRM)?
DRM prevents the unauthorized copying or redistribution of eBooks to protect the rights of authors and publishers. If you downloaded a book protected by DRM, you’ll get a license in the form of an .acsm file with your book. You'll then use Adobe Digital Editions (ADE) to add the DRM-protected content to your eReader.