Strong, up to date, look at the technology of bitcoin. The explanations of core concepts is good. The book covers a good selection of topics that are relevant in the current environment.
Hash functions, digital signatures, addresses, wallets, transactions, the blockchain, proof of work, peer-to-peer network, segregated witness, forks.
I think it is a good companion to Programming bitcoin as it has a completely different style that will be helpful to some people.