Doraemon teaches grammar. Meant for the last couple years of elementary school, to prepare children for middle school entrance exams. So it sums up all the basic points.
Foreign language acquisition is killed by textbooks, people studying any language, should try to ween off traditional textbooks, as fast as possible. In japanese it is not that easy to do so, because you need to have good kanji knowledge beforehand. But anyone with N3 should be able to read this book and enjoy it, without being swamped by unknown kanji and vocab.
A foreign language grammar is very different when you read grammat material meant for native speakers. It is enlightening and I've had my mind blown several times. Things made sense, puzzle pieces were put into place.
Plus it was fun! Doraemon and Nobita and Hime, making jokes along the way, this type of grammar teaching should be available in all languages. Complex linguistic terms presented in a very clear way, helps kids and language learners alike!
Not all parts will be useful if you are not a 12 year old japanese kid. For example the last parts about romaji and okurigana is not useful, but you'd still try and read through, pick up words and terms along the way.
Most useful parts: definitions of subject, predicates, modifiers,types of modifiers and syntactic dependency. Super useful parts: the sentence analysis chapter and the 10 parts of speech chapter. Also super useful, easily confused words (which covers the -rareru, the -souda and the -youni). See? Patience, even japanese kids have trouble with these.