Well I've been chatting with the guys around the office about Security books they have read and found useful. Below is a list of books you may want to get your teeth into:

.NET Framework Security
http://www.amazon.com/exec/obidos/ASIN/067232184X/qid=1098381226

Building Secure ASP.NET Applications: Authentication, Authorization, and Secure Communication
Online: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetsec/html/secnetlpmsdn.asp
PDF: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=055FF772-97FE-41B8-A58C-BF9C6593F25E

Improving Web Application Security: Threats and Countermeasures
Online: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetsec/html/ThreatCounter.asp?frame=true
PDF: http://www.microsoft.com/downloads/details.aspx?FamilyId=E9C4BFAA-AF88-4AA5-88D4-0DEA898C31B9&displaylang=en

Programming Windows Security
http://www.amazon.com/exec/obidos/ASIN/0201604426/qid%3D1098382062/002-4821233-3052860

Secrets and Lies : Digital Security in a Networked World
http://www.amazon.com/exec/obidos/tg/detail/-/0471453803/qid%3D1098381964/002-4821233-3052860

The .NET Developer's Guide to Windows Security
http://pluralsight.com/wiki/default.aspx/Keith.GuideBook.HomePage

Threat Modeling
http://www.microsoft.com/mspress/books/6892.asp

Writing Secure Code, Second Edition
http://www.microsoft.com/mspress/books/5957.asp

Happy Reading!