My brother has asked for a book to help him understand the Apple OS system and/or standard application programmes. He upgraded from Snow Leopard to El Capitan. (Whether he will upgrade to Sierra I don't know).
He has already told me not to buy the XXXXX for Dummies series.
This may be a silly reply but a lot of people miss the fact that Apple provides a 'proper' user manual for each Mac/OS version combination. They're not 'proper' in the paper sense but they're well written PDFs, running to many pages.
In addition to Tony's Apple manuals, O' Reilly are due to have their "macOS Sierra: The Missing Manual" by David Pogue and his team out in December if all goes well. It will be available in Paperback, and PDF/eBook formats.
This is the latest in the venerable series; they have been the only books that are useful in getting to grips with OSX/macOS beyond the simple basics. All the others I have seen run out of juice just when you need serious information. All are written with good humour for the non-geek, and are worth a look here.
A fairly large chunk of the book can be explored through the "Look Inside" PDF provided.
Rick Churchill 