If you VALUE your books, you want to KEEP YOUR OWN COPY.
Calibre is a GREAT way.
Things happen and the place you bought them from can:
Just Belly up (or the Author passes)
Lose the rights to distribute a Book/Author
3 of the stores I bought books from since I got my first reader (and joined MR), are just gone. Another's Author-Owner has heath + server issues and suspended service.
One no longer allows PURCHASES of some titles, but I can still download those I previously bought
Authors and Publishers. PLEASE have a
succession plan in place. Health issues can unfortunately, rear their ugly head at anytime