I got this book immediately after ordering my XSi. I was always a point and shoot (P&S) camera person before getting my first digital SRL camera. My wife had been wanting a DSRL for a while and I finally got one about 9 months ago. I became hooked on photography thanks in large part to this book, the XSi's capabilities, and having good subjects to photograph.
This book has everything you need to get up to speed and taking great pictures with your XSi. Let me make a couple of additional comments related to that statement. 1) A lot of the material in this book applies to other Canon cameras (and photography in general) but it is specific to the XSi features and layout. I've used what I learned from this book with my P&S Canon A590IS too. Never before this book had I used any settings other than the fully automatic ones on the P&S cameras. 2) Way better than the manual, which just matter-of-factly lists features. This book EXPLAINS the features and how they relate to one another (very important). 3) If you really get into photography and easily learn by reading a book, this is not the only book you will ever want/need. There are other books that get into the artistic aspects of photography you will likely want once you have mastered your camera, or at least become comfortable in the manual or semi-automatic modes.
What it doesn't cover: 1) Which specific lenses to get with your XSi. Very hard to answer so it's fair to be out of scope of a dummies book. So many reviews are conflicting on the topic anyway. 2) As mentioned, if you really want to get into artistic composition or advanced accessories, those are the topics of other books/websites. This is definitely the book to jumpstart your DSRL picture taking with the XSi. Well worth the price. If you are going to spend the money on a DSLR, make sure you get your money's worth. It pains me a little to see DSLR owners who use them exactly like they are P&S cameras with only the fully automatic modes of operation. The whole point of DSRL is to unleash the creative aspects offered by the different lenses and modes.
My summary recommendation: I keep this book in my backpack with all my camera gear. I haven't pulled it out in a while but it's good enough to carry around for reference if I want more information on a feature. There will always be another feature/setting you haven't fully explored and should experiment with more. If you get in a routine with your settings, try to add a new one to your bag of tricks. Taking great pictures with a DSRL is not always easy but it is very rewarding when you get those special shots you know you would not get with a P&S. Without this book, it would have taken me a lot longer to get proficient, and I would have missed out on some really memorable pictures along the way.