Easiest way on the Oasis, is via the oasis.
It'll allow you to just click, and queue the downloads.
It is one by one though, each screen having 7 books at a time.
Via a pc's web browser, you'll have to do it on the 'my devices' tab in
https://www.amazon.com/mn/dcw/myx.html , in batches of 10, but the work is more tedious.