Thoughts on using book of heros during Hammer jam?

I've 2 books from the current gold pass. My AQ is 83 going on 84. I'm torn between using the books to accelerate the upgrade

On one side I feel I can use the books on lower levels to maybe add 2 more levels during hammer jam while cost and time are 50%

On the other hand, I am thinking to save the books for later to be able to slash full upgrade time and get more value (but have to farm for full cost of upgrade)

Any thoughts? I have books of spells and buildings as well but that's an easier decision to save given there's only 1 level of upgrade. But its different for heros. Or am I missing something?