How to mark currently reading books easily?

Hi!
I am trying to figure out a way to mark certain books in my library (not that it matters, but simply those I am currently reading). I would like to be able to switch this mark on/off quickly, but I have more or less given up on this. Mostly I want a quick access to these books when I open Calibre so I think having them available in one of the "folders" in the left side menu (I don't think it has a name, so I marked it with red color on the attached jpg) might be the best. Although I can add a category there, I cannot add books into it and I don't know if that is even the best way to go about what I need.

If there's anyone with a nice idea about how to achieve anything like this, pls, let me know.

Many thank in advance for any kind help!

Best regards,
P

P.S.
I admit as much as I love the basic functionality of Calibre, I am frustrated with its complexity and some of it's UI logic. There is so much I need to research in my life to get things done at work, frustrating buggy software I need to work around and more and having to spend even more of the same time and effort on a book database... I just can't mentally go further there. But I wanted to admit I have not studied and understood the complexities of Calibre and I cannot at this point.

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