It’s essentially a GUI for Docker. It has its own quirks that, in my opinion, don’t match Docker’s UX and can make it more complicated to customize deployment. But Portainer can make multiple environments or dozens of containers easier to manage. e.g. I do most of the actual work using compose yamls but use Portainer as a reference to organize subnets, ports, and volumes. Or if I’m unsure what the problem is and just want to see all the details of each container faster.

Calibre is highly regarded and works well with nearly all e-readers afaik. It has a local server option for browsing, uploading/downloading, and community extensions.