Firstly, download Calibre, the de facto standard in ebook Library management software.

Install it as normal on your Mac or PC.

You can then use the Import button, or drag and drop files on Calibre.  When the files have been added to the Calibre library they actually copied, which means you can delete the downloaded ebook folder - it is no longer required.

You can read the ebooks directly on your computer, or send them to your device (apart from Apple devices - follow separate instructions for copying to them).

While Calibre is free, please consider a donation to support its ongoing development: