It's a classic book but as many others have mentioned in comments, a lot of red pillers use this book as a Bible of sorts, so it's gotten a bit of a bad rap.

In my early adulthood I was deep into MLMs and internet marketing and this book was the Bible, but it was a bit tautological because it was assumed that everyone respected and venerated that book, so all the marketing materials (that we had to purchase of course!) referred to the book.

Indeed, the best way to get rich quick is to sell get rich quick schemes.

On another note, an equally good book that is also used for manipulation is How to argue and win every time by late lawyer Gerry Spence. The book does what it says on the tin but it's more on persuasion methods and framing, which of course can be used for nefarious purposes.