Many Packt books are not free anymore (#3489)

free-programming-books.md

@@ -510,7 +510,6 @@
 * [Communicating Sequential Processes](http://www.usingcsp.com/cspbook.pdf) - Tony Hoare (PDF)
 * [Computer Musings](http://scpd.stanford.edu/knuth/index.jsp) (lectures by Donald Knuth)
 * [Confessions of an Unintentional CTO: Lessons in Growing a Web App](http://www.jackkinsella.ie/books/confessions_of_an_unintentional_cto) - Jack Kinsella
-* [Continuous Delivery and DevOps: A Quickstart guide](https://www.packtpub.com/packt/free-ebook/devops-quickstart) - Paul Swartout, Packt. (email address *requested*, not required)
 * [CS For All](https://www.cs.hmc.edu/csforall/)
 * [Culture \& Empire: Digital Revolution](http://hintjens.com/books) (PDF)
 * [Design With FontForge](http://designwithfontforge.com/en-US/index.html)
@@ -528,7 +527,6 @@
 * [Foundations of Programming](http://openmymind.net/FoundationsOfProgramming.pdf) (PDF)
 * [Game Programming Patterns](http://gameprogrammingpatterns.com) - Bob Nystrom
 * [Getting Real](http://gettingreal.37signals.com)
-* [Getting Started with Unity 5](https://www.packtpub.com/packt/free-ebook/getting-started-with-unity) - Dr. Edward Lavieri, Packt. (email address *requested*, not required)
 * [Hacknot: Essays on Software Development](http://www.lulu.com/shop/ed-johnson/hacknot-essays-on-software-development/ebook/product-17544641.html) - Ed Johnson
 * [High-Performance Scientific Computing](https://andreask.cs.illinois.edu/Teaching/HPCFall2012) (class lectures and slides)
 * [How Computers Work](http://www.fastchip.net/howcomputerswork/p1.html) - R. Young
@@ -540,7 +538,6 @@
 * [Learn Programming](https://progbook.org) - Antti Salonen
 * [Learn to Program](https://pine.fm/LearnToProgram/)
 * [Learning 30 Technologies in 30 Days: A Developer Challenge](https://blog.openshift.com/learning-30-technologies-in-30-days-a-developer-challenge/) - Shekhar Gulati
-* [Learning Docker](https://www.packtpub.com/packt/free-ebook/learning-docker) - Pethuru Raj, Jeeva S. Chelladhurai & Vinod Singh, Packt. (email address *requested*, not required)
 * [Linked Data Patterns: A pattern catalogue for modelling, publishing, and consuming Linked Data](http://patterns.dataincubator.org/book/) - Leigh Dodds, Ian Davis
 * [Magic Ink: Information Software and The Graphical Interface](http://worrydream.com/#!/MagicInk) - Bret Victor
 * [Mastering Bitcoin - Unlocking digital currencies](https://github.com/bitcoinbook/bitcoinbook) - Andreas M. Antonopoulos
@@ -555,8 +552,6 @@
 * [Philosophy of Computer Science](http://www.cse.buffalo.edu/~rapaport/Papers/phics.pdf) (PDF)
 * [PNG: The Definitive Guide](http://www.libpng.org/pub/png/book/)
 * [Pointers And Memory](http://cslibrary.stanford.edu/102/PointersAndMemory.pdf) (PDF)
-* [Practical Data Analysis](https://www.packtpub.com/packt/free-ebook/practical-data-analysis) - Hector Cuesta, Packt. (email address *requested*, not required)
-* [Practical Responsive Typography](https://www.packtpub.com/packt/free-ebook/practical-responsive-typography) - Dario Calonaci, Packt. (email address *requested*, not required)
 * [Principles of Computer System Design](http://ocw.mit.edu/resources/res-6-004-principles-of-computer-system-design-an-introduction-spring-2009/online-textbook/part_ii_open_5_0.pdf) (PDF)
 * [Procedural Content Generation in Games](http://pcgbook.com) - Noor Shaker, Julian Togelius, Mark Nelson
 * [Programming with Unicode](http://unicodebook.readthedocs.org)