Some of of these books are not "open source" [0]. For example, "The Boost C++ Libraries" book clearly gives a non-commercial license [1] as does "How to Think Like a Computer Scientist" [2]. I haven't checked them all but a lot of look to be under a free license [3] [4], so maybe the majority of them are actually under an open source license.

    [0] https://en.wikipedia.org/wiki/Open-source_license
    [1] https://theboostcpplibraries.com/
    [2] http://greenteapress.com/thinkcpp/index.html
    [3] http://opendatastructures.org/ods-cpp/About_this_document.html
    [4] http://www.network-theory.co.uk/about.html