Since this update:
books with long texts are broken. To create book like this use in a command block or the console:
give @p minecraft:writable_book 1 0 {title:"a",author:"abc",pages:["iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii12345"]}
If you try that in vanilla it will work, if you try it in spigot, the text is cut.
this also happens when you use the ingame minecraft book editor, fill the book with a lot of i's until you can input no more, remove about 10 characters, replace them with single chars each in one line, close the book, reopen it the last chars will be cut.
Before reopening the book:
After reopening:
To fix this i would recommend to increase the character limit for a page to 512 or more (maybe even 1024?) - that should be enough for "normal" texts - while 256 is not enough.
The page limit is also too small, by using commands you can create books with much more than 50 pages, this works perfectly in vanilla, but no longer in spigot.
In recent vanilla, created with a command block: