There was a problem creating books with click events.

If you add "give" to the commands.yml and use this syntax it will work.

/give @p minecraft:written_book 1 0 {display:

{Name:Book of Beef,Lore:[To Kill Cows]}

,title:'A Book',author:'Mactso',generation:0,pages:["{text:'Cowmoorticus',color:gold,hoverEvent:

{action:'show_text',value:'Click this text to kill nearby cows.'}

,clickEvent:

{action:'run_command',value:'/minecraft:kill @e[type=Cow,r=5]'}

,extra:[

{text:'Kill any nearby cows.',color:black}

]}"]}

It will also probably work with /minecraft:give.