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We bought this as a guide for us parents to know what a student must know by the time they are out of middle school. The book serves us like a "dictionary" for concepts explained simply - which we can then use as a starting point for teaching our kid... and using this as a skeleton, we supplement with videos, photos, online articles, etc. to extend the learning.
We have the other books in the series as well and they are just as excellent for the same reasons stated up here. Definitely a must-have for parents to know what their children should be knowing as well as the students themselves for simple explanations to vast variety of concepts.