When putting a book itemstack into a HashMap (as key) the following NPE occurs:
http://pastebin.com/frxT1uCi

Source code to the plugin which loads that itemstack from a config file just before attempting to store it in the said HashMap:
https://github.com/nisovin/Shopkeepers/blob/master/src/main/java/com/nisovin/shopkeepers/shoptypes/NormalPlayerShopkeeper.java#L216

And here how this unspecial book itemstack looks like when being stored in the config file: http://pastebin.com/NHEEU0xe

Though I assume it's not an issue with the plugin nor with the serialized itemstack, as this worked just fine on previous versions of spigot/craftbukkit.