Hello All,
I was going through almost all the Biztalk Server books present in the Market but the book which I would suggest are the best are:
1. Pro BizTalk 2006
by George Dunphy, Ahmed Metwally
2. Professional BizTalk Server 2006
by Darren Jefford, Kevin B. Smith, Ewan Fairweather
Pretty good books and well explained the Architecture of the Server along with many real time senarios.