You can try dual-n-back for free -> http://brainworkshop.sourceforge.net/
Luminosity is good if you pay (they have daul-n-back like challenge in premium), although I don't think the free version is that great.
I think it's less about any one book in particular than it's about reading as broadly, critically, and mindfully as possible.
By broadly I mean reading about as many different subjects as interest you, as well as those that don't interest you as much but you feel are important to know about.
By critically I mean actively reading material that challenges your beliefs, engaging with a variety of sources, and asking questions about the material as you go along.
By mindfully I mean, while reading, thinking about the many connections to other areas of interest and other things you've already read, as well as considering ways in which the reading affects the way you live and act in day to day life.
If I were to recommend any particular books, I think they'd be "Thinking, Fast and Slow" by Daniel Kahneman, "The Problems of Philosophy" by Bertrand Russell (as well as many of his other writings), "Meditations" by Marcus Aurelius, Plato's Dialogues, "A Short History of Nearly Everything" by Bill Bryson, "Freakonomics" and "Superfreakonomics", "Why Nations Fail", "Sapiens: A Brief History of Humankind"; you can't go wrong by exploring the classics as well, seeing what speaks to you and what doesn't (try Orwell and Huxley, Dostoevsky and Tolstoy, Hesse, Dickens, Joyce, Steinbeck, Vonnegut, and so on); finally, I'd recommend some poetry as it tends to get the creative aspects of the mind working in subtly different ways – my favorites are Roethke, Dickinson, Rilke, Plath, Oliver, Emerson, Rimbaud, Frost, Basho, Li Po.