Someone could overrule me but I don’t think this is doable in bubble. What I would do is create two additional fields, one that contains the text before the hyphen and one containing the text after the hyphen. Then when you sort you have two sort levels.
The book title is a user inserted value and I can’t define upfront any structure for the value, meaning it can be “6 Book”, “L6 Book”, “6_Book” whatever they might add…
Oh, then you might be out of luck. Depending on how important it is to you, you might need to write a custom plugin. Sounds like you know a little bit of javascript.
I was thinking about that and I don’t see a way around that. The logic for sorting an RG is built into the RG element and that code will not be open source. I think you’ll have to build your own from scratch.