You do not need to have the corresponding device. All the companies mentioned offer a version of their device to use for PC. This way you can download your book. BUT all the companies mentioned also have their DRM schemes. Which means that you will either be able to read said book on their device or PC version or you will have to strip the DRM which is not legal, but possible.
For others you will have to install
Adobe Digital Editions to get epub content. This is also needed to download books from ebook libraries such as the
Singapore Library that is for free!
There are shops that sell DRM free books. Some of them you will find
here for example. Also make sure to have a look at bewrite.net. Neil is a member here and they just giving away
two books for free for each of us.
For more information just use the Search function