I have the book, though I've not read the whole thing. It is basically a treatise on magick, covering both theory and practice. It also includes Crowley's Book of the Law.

The wikipedia page on the book provides a good summary of what you can expect to find in it: http://en.wikipedia.org/wiki/Magick_(Book_4)

You can read most of the book online here: http://www.hermetic.com/crowley/libers/lib4.html

And if you want to buy it, you can get it for around $60 on amazon.com.

It is a must if you are a Crowley fan. It is certainly a useful resource for those interested in magick, though there are other, I'd say better, books on the subject. I'd personally recommend Franz Bardon.