What term describes a person praising the content of book usually found in books foreword?

There isn't one specific term for a person praising the content of a book in a foreword. However, here are some terms you could use, depending on the context and tone:

Formal:

* Foreword writer

* Contributor

* Introduction writer

* Guest contributor

Informal:

* Endorser

* Reviewer

* Advocate

* Supporter

Other:

* Commendator (slightly formal)

* Lauder (more poetic)

You can also use phrases like:

* Someone who provides a glowing endorsement

* A person who writes a passionate introduction

* An individual who praises the book's merits

Ultimately, the best term will depend on the specific situation and the style of the foreword.