The intent of the book, as stated by the author, is: It assumes that you aren't looking for cryptographic theories, but practical ways to use cryptography

The intent of the book, as stated by the author, is: It assumes that you aren't looking for cryptographic theories, but practical ways to use cryptography in your projects. If you want to build a cryptographic library for other developers to consume, it's imperative that you understand the theory behind crypto principles. The author is telling you that this isn't the book for that.