It varies by section and need. Where a section has books of highly variable size, such as poetry, smaller ones stack horizontally in back while large ones are vertical in front. Where there's a lot of the same size, such as YA trade pb, it's more typically both inner and outer in the same orientation, with the most likely wanted books in front (such as DWJ).

(Dang it, I kleep clicking the wrong button.)

---L.