Really good book, but this book (as of today January 25 2008) is based on Houdini 8, which is not a good thing because the current Houdini 9 interface has been changed *a lot* since this book was written.
Sure, the underlying concepts are much the same, but in my humble opinion you gotta face this truth: when trying to learn a piece of software, the learning is much more efficient if the book you're using matches up closely with what you see on your screen.
Search the internet on "Digital Tutors" for two video-based CD-ROM titles out now that use the Houdini 9 interface. HDRI magazine issues 16 and 17 have some Houdini 9 based tutorials. 3D World magazine Issue 98 has a Houdini 9 tutorial, and the web site 3D Buzz has some tutorials.
Google on Side Effects Software Houdini and download the free edition to try out this amazing software. They even have a $100 version now that relaxes some of the more stringent limits on the free version. I don't work for them or know them, I just like Houdini a lot. I don't work for Digital Tutors or the magazines either, I just read them.
Oh, and while you're at the Side Effects Software site check out the link on Learning Materials to see about other Houdini 9 learning stuff.