One trick we're using which you might care to copy over (and improve upon) is a linear gradient to fake the pages. I think for thin books, I prefer your white version, but for thick books, it feels pretty sterile.
Test: set color to e.g. pink, increase radius to maximum value, see that the front image gets cropped but the pink area behind it is not. The backside of the book has a border-radius by the way.
Edit: might be nice to only apply it to the right-hand side as the side with the spine is unlikely to have rounded corners.
Would it be possible to add a different set of parameters that describe the physical book instead? What I mean is specifying the page size in some standard way (e.g. A3 or letter size) and the number of pages and let the algorithm decide the parameters of book thickness (in pixels) automatically.
The only comment I have is that the width slider tends to make an aggressively flickery mess of my screen anywhere above 250px.
I have no idea how well it works on various browsers (the bugaboo for all CSS), but it's noice!
excellent work otherwise
i love collecting little tools like this, lmk if anyone has one for mocking phones, tablets and desktops.
edit: please take this as a helpful suggestion and not a criticism. I still really like the work you've done.