There are a number of "How To" guides out there these days that offer to teach elements of a particular kind of art - sci-fi art, fantasy art, noir comics, etc. These guides focus largely on the nuance, etc., of the styles discussed and don't necessarily get into the fundamentals (i.e. anatomy, costume, etc. - which, rightly should be covered in other books). "Extreme Worlds" (I'll call it EW from here on) is one of these books, but unlike many of its predecessors, it does a decent job of highlighting the methods of modern sci-fi artists understandably.
If you're familiar with anything Wizards of the Coast has produced in recent years, then you've likely seen the artwork of EW's author, Francis Tsai. He's an excellent artist, working largely in digital media, and is an obvious choice for this book. As it is, EW has three sections:
1) Basic Concepts, in which tools of the trade (including digital media), shapes, perspective, color, etc. are discussed. By and large, 2-3 pages are devoted to each section, and aren't particularly deep, but enough to give an introduction to what is discussed. For instance, the "Color" section discusses the color wheel, and terms like "hue," "saturation" and "temperature" are discussed. A few artworks in this section are taken from start to finish to show how traditional and digital media "work." This section is probably the weakest of the three sections simply because it is the one that offers the least depth to the artist.
2) Demos, which shows the creation of characters (a humanoid, an alien and a robot), vehicles, and environments from start to finish. I found this particular section especially helpful, as it shows how Tsai goes through his creation process and also provides tutorials on paintings (although they're not especially detailed).
3) Space Opera, in which Tsai goes through the process of creating a sci-fi story-world and populating it with characters, transports, etc. At the end, he produces a sort of "movie poster" for it. This is an interesting section, mostly as it lets you see different kinds of art and provides some nice inspiration for your own projects.
The Good: There are some nice nuggets of gold here for the aspiring concept artist, however you'll have to read closely to get at them. The book itself is very attractive and well-printed (on glossy paper), filled with Tsai's artwork in various stages of completion, and reminiscent of his 100 Ways to Create Fantasy Figures, which is a similar book, but also worth a look.
The Bad: While this book calls itself a "complete guide," it isn't, really. A number of vital techniques are given short shrift (or not mentioned at all). A major concept art technique, working with grayscale markers (like MARKER SET-PRISM COOL GRAY) is not even covered. Also, issues like painting in digital media are focused on, but since it has become an industry standard technique, a full tutorial (possible included on a DVD-ROM, which a number of similar books have done) would have been great. Also, despite the basic information included in the first section, this book does seem to assume that you've had quite a bit of artistic background and at least have access to a computer and some version of Photoshop (or a similar program).
Overall, this is a great book for inspiration and tips on how to innovate your creative process. Books with sci-fi and fantasy art as a focus, but which are more tutorial-focused, include Fantasy Workshop: Mastering Digital Painting Techniques, Master Digital Color: Styles Tools Techniques and Digital Character Painting Using Photoshop CS3 (Graphics Series), which might be more helpful from a "How To" perspective (and 2 of which come with tutorial DVDs).