a request out of the blue from Packt to write a book for them is unsolicited commercial email
The poster asked for a review on Amazon in exchange for an e-copy of any book of your choice, not write an entire book. And this is not "out of the blue". The publishing of this book is a milestone for the software, it's on the timeline for our Facebook page.
If you don't like his email, then ignore it. He wants the book to be successful and we want the project to be successful, I see synergies there.
If the author of LMMS is in the credits of this book, perhaps there's some good we can take from getting pinged from the publisher.