link to ebook (MicrosoftDocs#451)

Marketing requested that we cross link to the ebook.

docs/guide/index.md

@@ -42,7 +42,7 @@ Immutable infrastructure<br/>
 </tbody>
 </table>
 
-This guide is intended for application architects, developers, and operations teams. It's not a how-to guide for using individual Azure services. After reading this guide, you will understand the architectural patterns and best practices to apply when building on the Azure cloud platform.
+This guide is intended for application architects, developers, and operations teams. It's not a how-to guide for using individual Azure services. After reading this guide, you will understand the architectural patterns and best practices to apply when building on the Azure cloud platform. You can also download an [e-book version of the guide][ebook].
 
 ## How this guide is structured
 
@@ -74,6 +74,7 @@ The Azure Application Architecture Guide is organized as a series of steps, from
 [checklists]: ../checklist/index.md
 [compute-options]: ./technology-choices/compute-comparison.md
 [design-principles]: ./design-principles/index.md
+[ebook]: https://azure.microsoft.com/campaigns/cloud-application-architecture-guide/
 [patterns]: ../patterns/index.md?toc=/azure/architecture/guide/toc.json
 [pillars]: ./pillars.md
 [ref-archs]: ../reference-architectures/index.md