Bronshtein, Semendyayev, Musiol, Mühlig: Handbook of Mathematics, 6th Edition – nowadays (resp. for quite some while) there have been 2 parallel editions (plus translations) of this classic math book As of 2015-04-02 you can’t even purchase the 6th edition as paperback – but there are sources, where you get it as PDF. Isn’t that appalling?