With a game like Animal Crossing where content post launch is still being developed and deployed, it is inevitable that the guide will arrive already incomplete.
Having said that, in my opinion it is still a very useful book.
Sure all the info you find in this book will will find around on the internet, but the book is very well written, and packed with information.
It does not have information on Redd and paintings, or leif and his garden products, but it has everything else.
The book is very organized, from basic tips like how to start the game, to advanced stuff like flower crossbreeding, house decoration, etca.
What impressed me the most is that sure, i do have all that info somewhere else, but in every regard, i find that i always read and learned some details i didnt find on the internet before.
So overall it is a very useful guide, but most importantly, if you are a big fan of the game, it is a great item to have in your collection.
TL/DR: you dont need it, but it is awsome to have!