In short, HtDP is a book about programming, whereas PLAI is a book about programming languages. You can think of PLAI as teaching you how to implement the tools

In short, HtDP is a book about programming, whereas PLAI is a book about programming languages. You can think of PLAI as teaching you how to implement the tools you learn how to use in HtDP. They both come from members of the PLT research group, so share a basic philosophy. In fact you use the very same tools (mostly structural recursion) in implementing languages that you learned in HtDP.