You can get it for free. It's basically a series of blog posts (by a number of authors) which tries to explain some of the technical workings of CouchDB, and a few neat things you can do with it. It's kind of deep, but not comprehensive enough, and doesn't really cover the basic practicalities very well.
The wiki is much better if you just want to get stuff working. The book is more for background.
The big issue is they don't seem to treat the documentation and client libraries as a really big deal. Look at a language like Python - it's nothing special, but it's great tutorial and documentation gets it a lot of mindshare. Heck, look at Ruby - it's Python with a bit of syntactic sugar, less libraries, and a slow interpreter. But its documentation (which is great, but offbeat) wins it rabid fans like DHH, who go on to make stuff like Rails, and it's practically a household name.
There's some really neat stuff that only CoucbDB can do, but figuring out how to use it is just a pain.