Microsoft Patterns & Practices released the 2nd edition of their Microsoft Application Architecture Guide. You can read it for free via HTML or purchase a copy from Amazon.

If you are using Microsoft development technology for example, Visual Studio as your development tool in your working or your part time job, you probably hear about Enterprise Library. It is best practice framework for building enterprise application for either Web or Winform application.

There is release on 2nd edition of their Microsoft Application Architecture Guide. Check it out.