Flavio is a prolific writer and educator whose blog I benefited from often on my journey to learn how to code. I picked it up alongside others on the topic after I already made apps with Next.js because there's often a gap between tutorials that teach you how to do things step-by-step, and knowing why things are the way they are, or why you're doing the things you're doing if you come at it fresh.
On the plus side, the book is free and helps go beyond some of the documentation to explain some of the finer aspects of next.js app-building outside the create-next-app sandbox where things are configured for you. It's very clear and concise, particularly as it's hard to do technical writing in English to begin with, never mind when it's not your first language.
The downsides were that in some explanations didn't get key concepts across to help you form mental models, which is important when learning new concepts. For instance, there's the claim that next.js handles prefetching 'transparently'. It's the opposite of transparent, since this is handled behind the scenes for you - and he even explains what goes down out of sight and out of mind.
The other is that some of the other phrasing could've been improved to make it less dry, in a format that helps get across why you'd want to do one thing or the other. For example, creating a common layout template for your site. All the steps in the tutorial are stated as facts, but imo instilling a sense of curiosity ("how might I do X with this technology?") would be better, since it's a book aimed at beginners.
Already a bit outdated as Next.js introduced some new features that revolves around data fetching and router usage. All in all a worthwhile read for a total beginner. Would probably get more in depth step by step guide from zero to beginner with the official Next.js docs though.