This "book" seems very questionable, for a number of reasons:
- There is no author listed or even ever mentioned. Most non-scam badminton books are written by sport scientists, coaches or players, none of whom would have a problem with their name being associated with a book they wrote.
The domain is registered to Reuben Lau, a Canadian web developer with no significant badminton qualifications I could find.
- The testimonials are likely fake: None of them mention even first names. The style of language is quite similar overall.
- Smash secrets is something many beginners would look for, when more advanced players understand that badminton is more complex; an important part of a smash is the footwork. Therefore, most legitimate badminton books cover the whole technique.
- The first chapter seems to focus on rackets. This is a surprising start. Most actual badminton books don't have more than a small discussion about rackets.
- Chapter 3 covers jump smash - which seems sensible - but also backhand smash. The technique for a backhand smash is totally different than any forehand shot. This grouping seems rather strange. Most actual badminton books have one chapter for overhead forehand shots (clear/smash/drop) and one for overhand backhand shots (drop/clear/smash).
- It's self-published, which is not that rare but does not bolster legitimacy.
Without having bought the book, these red flags would prevent me from buying it. Instead, better ask a local coach to do a smash-only session for you.