I suggest looking at
Books on Board help. It has generic Adobe help, and the Astak device help should also apply to most devices with mobile Adobe Digital Editions.
If the existing ebooks are in multiple folders, then the device supports folder navigation. Some devices require ebooks to be in particular primary folders (e.g. Documents or eBooks, as opposed to Music), but usually this isn't the case. Most likely you can make whatever folders you want and copy ebooks, even ebooks with Adobe DRM, anywhere you want (into internal memory or the SD card) via USB. You can also let Desktop Adobe Digital Editions copy your Adobe ebooks over for you, but then they will all end up in one particular folder.