In what way is your current Kindle insufficient for your needs?
If it has stopped working, any new Kindle can do anything the old one could do.
Really, you can use any ereader on the market whatsoever, whether app or e-ink device. No matter what format it is in, the ereader will either be able to read it, or else
calibre can convert it into a suitable format.
(You may want to own every single device on the market and check the finished product on
all of them, for various
other reasons. Just ask Hitch.
)