I don't know all that much about PocketBook, but to reopen your last document use the hamburger menu in the top right.
To know what the nightly builds contain you can check on GitHub.
There are the releases with release notes:
https://github.com/koreader/koreader/releases
And then on top of the release it'll say something like "6 commits to master since this release."
If you click that link (
here) you get a complete list of every change that's been made.