Book clickEvent has the same maximum character number like the chat.

If you have a JSON-book with a command that has more than 100 characters the command will get cut off when you try to use it in both 1.8.8 and the last Snapshot (15w35b)
When it comes to NBT-Data almost alleays you'll need more than 100 characters so this is really unfortunately.
For example the fallowing command will get cut of in the chat at "Da" of "Damage":

/setblock ~ ~ ~ minecraft:chest 0 replace 
{
	CustomName:test,
	Items:
	[
		{
			id:diamond_chestplate,
			Count:1b,
			Damage:500s
		}
	]
}

The same command inside a clickEvent of a book:

/give @p minecraft:written_book 1 0 
{
	author:"me",
	title:"testbook",
	pages:
	[
		"{
			\"text\":\"test command\",
			\"clickEvent\":
			{
				\"action\":\"run_command\",
				\"value\":\"/setblock ~ ~ ~ minecraft:chest 0 replace {CustomName:test,Items:[{id:diamond_chestplate,Count:1b,Damage:500s}]}\"
			}
		}"
	]
}

Will return: "Data tag parsing failed: Unbalanced brackets: {CustomName:test,Items:[{id:diamond_chestplate,Count:1b,Da"
As you can see Minecraft only got the command to "Da".
The command works if you leave out the "Damage:500s", though.

Whilst I was searching for the issue already been posted, I found only a "fixed" issue. It might be that this issue was fixed during the 1.8 Snapshots and came back in a later one. MC-47060