You can also between computer and ereader device ADE.
If you have two devices/computers associated with the same AdobeID you can synchronize the books by copying the files outside the Adobe provided infrastructure;
for example using a file manager or some syncing software.
You cannot use DRM books associated with one account on another account.
You can copy them as above, but you won't be able to read them an a device that is not associated to the same account that the book is associated with.