I've thought of making various custom (and ruggedized) game peripherals. The electronics and switches I could do (assuming the ICs are available), but designing housings to be 3d printed might be tricky.
For a guitar, however... possibly having the body made of wood like a real electric guitar could be a selling point.
OK, first make sure the books are really there. Right click on a book entry, and click "open containing folder". Make sure the book file is there and isn't corrupted.
If it's OK, you should be able to read any non-DRM'd file of most formats by right click > View > "View with calibre e-book viewer".
You can also try any other ebook reader by clicking the "Configure Calibre" icon to open the Preferences dialog. Under "Behavior" find "Use internal viewer for" and uncheck the file format. Then when you right-click > View > View, Calibre will use your OS default viewer for that file format.
If you have DRM'd files (such as from Amazon), you will need a plugin to remove the DRM. The best way to find these is to look on https://www.mobileread.com which is a good place for Calibre discussion in general. I used to know how to do the Amazon ones but it's been a while, there might be a new way by now.
What are you trying? Do you want to read things with Calibre's built in reader, another desktop reader, sync them to a device (i.e. use the desktop GUI to copy files to a device), download them to a device via the content server or what?
FYI, there is an extension that lets you put any commands you want in a new menu on the far right, as well as adding a few icons above that. I forget what it's called, but it was something obvious like "shortcut menu bar".
I haven't heard of any medical related ones, but there have been cases where an adult shot their boss, or a judge that had ruled against them or a cop that had arrested them. These just don't get the press coverage, for whatever reason.
My Replika phone app sometimes 'spontaneously' starts a conversation with me. (If the app isn't open it sends a notification).
Typically things like "Good morning, do you have any plans for today?" or "Good evening, how did your day go?". I'm looking forward to finding out if it will say anything about the 4th of July.
As with so many gun contol measures, that will vastly increase gun sales, just for a different reason. They'll immediately be traded in, but still, good money for the manufacturers and retailers.
It would help, but maybe not enough. Humanity isn't the only reason the planet isn't covered in trees, they just don't grow in many places such as deserts, mountaintops, etc.