Newsletter subscribers can be a clean source of early book reviews because they chose to hear from you. They are readers first, not people trading favors.
The safe rule is simple: ask for an honest review, make it optional, and do not reward the review.
How To Ask
Send a short note after launch. Tell readers that if they read the book, an honest Amazon review can help other readers decide whether the book is right for them.
Do not ask for five stars. Do not offer gifts, refunds, or bonus content for posting. Do not tell readers what to write.
What To Avoid
Avoid sending everyone the same script to copy. Avoid pushing only happy subscribers to Amazon while sending unhappy readers somewhere else. Avoid daily reminders.
One clear request and one later reminder is enough.
Better Signal
A review from a real subscriber who owes you nothing is stronger than a review from a friend or a swap partner.
Use the newsletter to reach readers, not to force praise.