Sadly, I think this book is now obsolete, but also, it has some issues.
What it says about GraphQL I believe is correct, but there are new features that are not included, such as subscriptions. In terms of a concise explanation I believe Learning GraphQL has a better one for GraphQL and it's also more complete (just because it's a later book).
In terms of Relay, I couldn't recognize anything from the book as compared to what I'm seeing on Relay's website and documentation. I haven't spent a lot of time researching why this is the case, but I suspect that Relay's API has been greatly changed and improved since this book was released.
Aside from that, around the Relay chapters, there are leftover editorial marks that always reduce my trust in the content.