Thanks for pointers. I looked how to create a junction and I found this small app: https://schinagl.priv.at/nt/hardlinkshellext/linkshellextens..., it's a GUI for creating symbolic links/junctions/etc. Which for me is great as I'm not an avid user of cli.
Does it support syncing local folders? For example a game put saves in Documents folder and I would to sync it to my Dropbox folder, is such thing possible?
I've used Type Faster and Sublime Text to achieve something like this but instead of books I've used articles/tutorials/documentation or articles in another language. In Type Faster I was creating new lessons with article I wanted to read. In Sublime Text I've used two panels - on left side I had article on right it was my typing space. While Type Faster shows you what word you have to type right now Sublime doesn't so I had to memorize as much words I wanted to type to avoid looking back and forth between panels. This is also useful when learning new language, if you are using Duolingo you most likely focus on speaking/reading/pronouncing words and not on writing so retyping articles in that language is very good method to memorize words.