Get both 
There is a coupon code for Ben’s book here and if you subscribe to Manning’s mailing list they also offer discounts from time to time. I buy around 80% of my books when voucher’s surface, even if I know it will be months before I can read them 
We are very lucky in Elixir that we have so many great books - other languages of a similar age don’t have any published books yet! A well-written, professionally edited and published book (especially from a tech publisher) is worth 50 mediocre books imo (something you often get when people decide to self-publish, not always but often).
