What book(s) do you recommend before Concrete Mathematics?
Is something like "Introduction to discrete Mathematics" enough?
It would help to know what math classes you've covered thus far. Knowing some basics in calculus will certainly help. From one of your comments below your question, it seems as though you haven't had any calculus, it would be good to cover some of the material typically covered in a calculus survey course, or 1st semester calculus. Spivak's Calculus was one of the recommended texts; you could also learn calculus, or supplement your study of a text, with on-line video-lectures and tutoring: I highly recommend that you visit The Khan Academy website: everything offered on the site is free, and it has built a great deal of credibility (e.g. Bill Gates has offered to sponsor the site.) Not only does the site offer lessons and exercises in calculus; it also covers geometry, trigonometry, and algebra. It is very extensive and a good resource for "brushing up" on previous learning too.
Now, from my take of the preface to Concrete Mathematics (see link to the book's preface below), the book was, by design, written to make the content accessible to a "wider audience (including [college] sophomores)", and hence, doesn't seem to assume any intensive background in college math. So, I think you could probably "jump right into" the book; if you do encounter any difficulties along the way, the book has an extensive bibliography to which you can refer, or a quick web search (Google, Wikipedia, MathWorld, etc.) of the topic causing you problems will turn up lots of resources to help you out.
However, if you are really unsure of your capacity to master Concrete Mathematics at this point in time, then by all means, prepare using some of the suggestions cited here. Discrete math might provide some preparation prior to reading Concrete Mathematics, but it seems to me that the relevant content from discrete mathematics is covered in the text. It certainly wouldn't hurt to study discrete mathematics (perhaps take a look at Kenneth Rosen's Discrete Mathematics and its Applications); it all depends on the time-frame you have available, your level of commitment, your capacity to stay focused, and the confidence you have in your abilities.
For those who aren't familiar with the book Concrete Mathematics, an overview of the text can be found here. To save you some "web surfing time", I'll quote from that webpage below:
"...Concrete Mathematics is a blending of CONtinuous and disCRETE mathematics. "More concretely," the authors explain, "it is the controlled manipulation of mathematical formulas, using a collection of techniques for solving problems." The subject matter is primarily an expansion of the Mathematical Preliminaries section in Knuth's classic Art of Computer Programming, but the style of presentation is more leisurely, and individual topics are covered more deeply. Several new topics have been added, and the most significant ideas have been traced to their historical roots. The book includes more than 500 exercises, divided into six categories. Complete answers are provided for all exercises, except research problems, making the book particularly valuable for self-study..."
My own feeling on this is that if you are an undergraduate in an engineering discipline, with the usual math sequence behind you (e.g. calculus I,II,III, & linear algebra), you will have a difficult time with Concrete Mathematics.
You would be much better served starting with an introductory text (I think Epp's Discrete Mathematics with Applications is excellent), and afterwards going to that book.
Concrete Mathematics is an amazing book, but it assumes you already know the basics that would be taught in a 1-semester course on the subject. The problems are superbly gauged, and even the answers (which are provided for all the exercises) often require significant thought to understand. I am currently halfway through it and am just amazed at its density, how well written it is, and how perfectly the answers help one to solve the respective exercise without just giving the result away.
But I have already been exposed to the discrete math basics and still find Concrete Mathematics is hard work.
The preface may imply it can be read by a wide audience - there may be college sophomores in engineering programs who can handle this material. But they would be the rare exception.