I just wrote integrations with the Twitter API, Facebook Connect, and Myspace's REST API. Twitter was by far the easiest, and is dead simple. Save your money and just read the API doc, and then use one of the many libraries someone else already wrote.
I played around with the Twitter API yesterday for the first time and running HTTP REST requests via curl (install it with Cygwin on windows) was invaluable to tracking down some problems and figuring out the expected results
The best way in my opinion to learn Twitter API is to get involved by reading the API wiki ( http://apiwiki.twitter.com/ ), join the google group for Twitter API.
I've had to deal with the twitter API twice, and both times a good way was to read their API docs. It takes maybe 20 minutes to read the whole thing and wrap your head around it. It's like not you're implementing NFS, the X protocol, or interfacing with an XMPP server.
Not sure how good your feedback is going to be here, it doesn't look like that comes out until April 2009. Kind of confusing considering it says Buy Now though...
