set it on its side：

set是“放置”，it 指“盒子”。on its side 指 “放置的方式”，作“方式状语”，不是地点状语。意思是“一侧放置；不是正面朝上”。

So I got a pasterboard box from the alley, set it on its side by the lamppost and put my papers inside. 

【翻译】于是我从小巷里找了一个石膏板盒子，把它侧放在灯柱旁边，把我的报纸放进里面。

on its side 这个用法，是“方式状语”，不是“地点状语”。我再举几个类似的例句：

He stood on his head to make the baby laugh. 他倒立着使婴儿发笑。  

He lay on his back, his hands behind his head. 他仰卧着，双手扣着后脑勺。

He lay on his stomach, making an earnest business of sleeping. 他这会儿趴在床上，看样子好象睡得正熟呢。