I think I found a way. You can get the bounding boxes for the books and then calculate their dimension with the accumulate node. Then you can transfer the size back to the books via their index.
I kinda hope I’m overcomplicating something because this was way harder to figure out that I anticipated. Debugging fields can be bit opaque…
Here’s the file. Maybe someone can find a simpler way to do this!
Stack of books.blend (923.1 KB)