The excellent Professional BizTalk Server 2006 by Darren Jefford (Author), Kevin B. Smith (Author), Ewan Fairweather (Author) is now available at all good book shops. In fact here's a link to it at Amazon
The book is an excellent reference for any would be BizTalk developer. I know the authors both personally and by reputation and can vouch for their creditability. I you were to buy only one book on the topic this is it!