Of all the books on octonions, division algebras, Clifford algebras, and related fields, none is more precise, theoretically deep, mathematically clear and logically organized and coherent and concise than Okubo's book. It requires more mathematical sophistication than most books on the subject, but any reader should be able to hire a consultant or tutor specializing in mathematical physics or even advanced algebra to clarify in ordinary language or an approximation thereof what Okubo is saying. A reader of Okubo's book will obtain a deeper insight into the mathematics of these algebras than from any other book, including the relationships between different types of algebras such as Hurwitz and non-Hurwitz algebras. The book is a little short on physical applications, although there are enough to provide impetus for reading other books, but the mathematics is absolutely profound and should not be missed by anyone. I have cited Okubo in papers at conferences and publications of mine.