I have been a software programmer for over six years and I'm from a non-mathematical background. Though I had some limited exposure to discrete mathematics in my college years it didn't leave any significant impact on me; but now I have been finding many topics on discrete mathematics to be very interesting, especially combinatorics and I'm interested in learning more of it.
However few people I had approached told me that I need to first master calculus in order to take on discrete math, as they say that's how it is taught in universities, is this true? do I need know calculus to appreciate discrete math? (Also, can anyone point me to any books on combinatorics for beginners)