This is a good place to start, at least for using the command line:
In the Conclusion section, there’s a link to download a massive free PDF reference manual, too.
Ubuntu and PureOS are both based on Debian, FYI, so the same commands are used on them all.
As for GUI usage, it’s just point-and-click, as with any modern desktop, although many applications, although installable on the Librem 5, might not be usable on the small screen, and might not launch at all, due to the different software architecture (arm-based CPU).
You might like this Tips & Tricks guide (some of which is no longer necessary since the repository* change from PureOS amber to PureOS byzantium) and this, curated list of tested apps as well.
And of course there are several actual Linux for Dummies books out there.
*repository - “store” from where software applications are downloaded