My memory use goes from under 300MB up to over 400MB after using a linking book only three times. And because I have memory limited to 512MB I get huge lag as the memory is not freed and Java keeps freeing the little memory I have left every 5 seconds. Kind of very annoying. There is no other way to fix it than restarting whole Minecraft, the memory is not freed by reloading the world. F3+A reloads chunks also which doesn't help either.