Hi Derek,
I asked this exact question to a local usergroup about a week ago.
As a result, these are the books I bought from Amazon (I wanted several perspectives aswell, as I find not all books cover all parts of a topic equally, or equally well).
".NET Security Programming (Gearhead Press - In the Trenches)" Donis Marshall; Paperback;
"CLR via C#, Second Edition (Pro Developer)" Jeffrey Richter; Paperback;
"Programming .NET Security" Adam Freeman; Paperback; ".NET Framework Security" Brian A. LaMacchia; Paperback;
HTH - Adam