Update book pricing to .99 + add PE Book countdown banner

Removes the launch-week '$0.99' messaging from the RAG hero (book is now at steady-state .99 / Kindle Unlimited). Adds a new bracketed banner section advertising the companion Prompt Engineering book on Kindle Countdown Deal at $2.99 through April 21.

The PE countdown banner uses the same click-tracker URL pattern as the rest of the README. After Apr 21 the PE banner can be removed in a one-line revert.

Existing book image, title link, and 'Get the book on Amazon' CTA all retain their click-tracker routing and Amazon Associates affiliate tag (diamantai-ragrm-20).

Author: NirDiamant

README.md

@@ -10,18 +10,30 @@
 
 Built on top of everything in this repo, the book goes far deeper: the **intuition** behind every technique, **side-by-side comparisons** of when each one wins (and when it quietly fails), and **illustrations** that make the tricky parts finally click.
 
-### ⏳ **Free with Kindle Unlimited** or **$0.99** launch price
+### **Free with Kindle Unlimited** or **$9.99** on Amazon
 
-The price goes up once the launch window closes. Readers who grab it now lock in the lowest price it will ever have, and get the same material that took months of research, writing, and iteration to put together.
+### 👉 [**Get the book on Amazon**](https://europe-west1-rag-techniques-views-tracker.cloudfunctions.net/rag-techniques-tracker?notebook=main-readme&click=book-buy-amazon-cta&target=https%3A%2F%2Fwww.amazon.com%2Fdp%2FB0D76734SZ%3Ftag%3Ddiamantai-ragrm-20&text=Get%20the%20book%20on%20Amazon)
 
-### 👉 [**Get the book on Amazon before the price changes**](https://europe-west1-rag-techniques-views-tracker.cloudfunctions.net/rag-techniques-tracker?notebook=main-readme&click=book-buy-amazon-cta&target=https%3A%2F%2Fwww.amazon.com%2Fdp%2FB0D76734SZ%3Ftag%3Ddiamantai-ragrm-20&text=Get%20the%20book%20on%20Amazon%20before%20the%20price%20changes)
+</div>
+
+---
+
+
+<div align="center">
+
+### 🔥 This week only: companion book on Kindle Countdown Deal
+
+The series prerequisite, **Prompt Engineering: Master the Art of AI Interaction**, is on a Kindle Countdown Deal at **$2.99** through April 21. Strengthen your prompting foundations before working through the RAG techniques.
+
+### 👉 [**Get Prompt Engineering at countdown pricing**](https://europe-west1-rag-techniques-views-tracker.cloudfunctions.net/rag-techniques-tracker?notebook=main-readme&click=book-buy-pe-countdown&target=https%3A%2F%2Fwww.amazon.com%2Fdp%2FB0DZ85RPB5%3Ftag%3Ddiamantai-ragrm-20&text=Get%20Prompt%20Engineering%20at%20countdown%20pricing)
 
 </div>
 
 ---
 
 
 
+
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