Happy news for all the developers around the world, we have released the Patterns and Practices guidance on Application Architecture. It is 381 paged PDF book and freely available from http://www.codeplex.com/AppArchGuide.
This books is majorly divided into four parts,
Part I, Fundamentals
Part II, Design
Part III, Layers
Part IV, Archetypes
and forwarded by S. Somasegar and Scott Guthrie. So enjoy.