The initial source comes from https://github.com/sdcuike/issueBlog/issues/4
https://github.com/PacktPublishing free to download books code by Packet
https://github.com/EbookFoundation/free-programming-books Very immense
http://www.ituring.com.cn/book?tab=free Turing free books
http://ebook3000.com/Programming/index.html Not just it ebooks, including other areas, has ads
http://ebook-dl.com Either download or see films, interesting
https://scanlibs.com BAD ! download interval time is restricted
http://www.java1234.com/a/javabook/ Chinese site
https://www.iteblog.com/archives/category/ebooks/ focus on big data
https://www.packtpub.com Publisher
https://www.packtpub.com/packt/offers/free-learning One free book per day
http://www.oreilly.com/data/free/archive.html
https://www.pdfdrive.com Very Good ! many areas
http://ebook3000.com/Programming/index.html
http://proglang.informatik.uni-freiburg.de/teaching/concurrency/2014ws/
http://www.colorado.edu/cs/computer-science-course-archive
http://mindview.net/Books/DownloadSites/
http://inventwithpython.com/#invent
http://www.mhprofessional.com/
https://github.com/justjavac/free-programming-books-zh_CN
http://www.ituring.com.cn/book/ebook
http://itmanagersinbox.com/1875/13-sites-to-download-free-ebooks/
http://www.lemis.com/grog/Documentation/Lions/
http://www.tcpipguide.com/free/t_TCPConnectionTermination-2.htm
http://www.freesoft.org/CIE/Course/Section4/11.htm
https://landing.google.com/sre/book/
https://bushkarl.gitbooks.io/spring-cloud/content/
http://www.sonatype.org/nexus/resources/resources-book-links-and-downloads/
https://books.sonatype.com/mvnref-book/reference/index.html
http://book.huihoo.com/maven-the-definitive-guide/index.html
http://www.commandprompt.com/ppbook/ Practical PostgreSQL
http://use-the-index-luke.com/sql/table-of-contents SQL Performance Explained
http://math.hws.edu/javanotes/
http://aosabook.org/en/index.html The Architecture of Open Source Applications