A thread is a set of posts with a title, like "New member and a few questions please". Threads don't have sub-forums. I don't think you can subscribe to a whole forum, just individual threads. A forum is a set of posts, like "General Discussions".
To subscribe to a thread, click on the "Thread Tools" button at the beginning of the page (on the menu bar with "Search This Thread", etc.), and select "Subscribe to this thread".
Books from B&N will have DRM, and would have to have that stripped before you could convert the book.
I don't know about books in the Calibre shop, but I assume someone will. You can always ask in the Calibre forum.
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