Cache question re: Calibre Library Android app

Hello,

I have Calibre Library installed on my Google Nexus 7; I've only used it once (so far) to place an epub file from my PC into the Nook folder on the N7 -- and it worked flawlessly.


Using a file explorer app on the N7, I noticed that the "Cache" folder has dozens of files all in the format "-21449....." What are these files? And are they necessary? (i.e. can they be deleted?) And if not necessary, why are they there?


Thanks for your time and response.
Cheers,
- martyh