Add Confluent's free ebook: Designing Event Driven Systems by Ben Stopford (#8434)

* Add Confluent's free ebook: Designing Event Driven Systems by Ben Stopford

* Add missing dash in the title

books/free-programming-books-subjects.md

@@ -774,6 +774,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Building Secure & Reliable Systems](https://static.googleusercontent.com/media/landing.google.com/en//sre/static/pdf/Building_Secure_and_Reliable_Systems.pdf) - Heather Adkins, Betsy Beyer, Paul Blankinship, Piotr Lewandowski, Ana Oprea, Adam Stubblefield (PDF)
 * [Code Simplicity: The Fundamentals of Software](https://www.codesimplicity.com/book.pdf) - Max Kanat-Alexander (PDF)
 * [Data-Oriented Design](http://www.dataorienteddesign.com/dodmain/dodmain.html)
+* [Designing Event-Driven Systems. Concepts and Patterns for Streaming Services with Apache Kafka](https://assets.confluent.io/m/7a91acf41502a75e/original/20180328-EB-Confluent_Designing_Event_Driven_Systems.pdf) - Ben Stopford (PDF)
 * [Developing Reactive Microservices](https://info.lightbend.com/COLL-20XX-Developing-Reactive-Microservices_Landing-Page.html) (email address *requested*, not required)
 * [Domain Driven Design Quickly](http://www.infoq.com/minibooks/domain-driven-design-quickly)
 * [Domain-Driven Design Reference](https://www.domainlanguage.com/ddd/reference) - Eric Evans