(but I don't feel particularly strongly about it and there may be other good, or even better, choices)R: http://nostarch.com/artofr.htm
(definitely the only good choice for someone w/ a programming background, the others are mostly poor-quality, boring cookbooks in comparison and will not teach you R programming)If you already know C++ well I think there's a limited gain in picking up C#, better spend time on improving your C++ (how's your knowledge of C++11 (incl. but not limited to async/futures/promises), Boost (incl. but not limited to Boost.Asio, Boost.Lockfree), cpp-netlib and/or POCO?) and learning the remaining languages.