ebook-reader permissions - no explanation given

Hello all,

I was looking for a simple(!) ebook reader, and found the below :

https://f-droid.org/en/packages/com.github.axet.bookreader/

Ofcourse, I checked its permissions - which I now have questions about :

"read the contents of your shared storage
  Allows the app to read the contents of your shared storage."

Allright, otherwise it can't read any EPUB files.

"modify or delete the contents of your shared storage
  Allows the app to write the contents of your shared storage."

No idea why it would need to do that(1).

"read phone status and identity
  Allows the app to access the phone features of the device. This permission
allows the app to determine the phone number and device IDs, whether a call
is active, and the remote number connected by a call."

I can make a couple of guesses to why it would need this, but that doesn't
help me in the slightest.  Does any one have a (non-guessed) idea to why I
should allow this ?

(1) Assuming that this is needed to store some configuration (last opened
EPUB, position therein), isn't there some per-app protected storage
available ?
I'm not really happy with the idea that that app would be able to muck
around with files that are his.

Remark: My smartphone runs "Graphene OS", which gives a nice indication
about how I appreciate apps which regard my whole smartphone as their
playground.

I did find some other ebook readers, but as those wanted a*lot* of
permissions (including network, so the app builder could gather "statistical
data" - his words, not mine) I discarded them on sight.

Regards,
Rudy Wieser