Database Connection Pooling
If you are using SQL Server then database connection pooling will be done for you automatically. Unless you have an extremely high transaction rate, this shouldn't be a concern.
Design, Coding, Testing, etc. can really be answered on two levels.
If you are looking for resources for Agile, for example, (the "meta" level) then Robert Martin's works are generally considered the reference. He has one called "Agile Principles, Patterns and Practices in C#" that is good if you are a C# developer. I can recommend it although I will tell you that it is pretty derivative of earlier works and his C# isn't really world class (see Jon Skeet's book, mentioned below, for that). Martin's book is very useful for learning Agile methods and design patterns but not so much for learning coding techniques.
For more of an algorithm-at-a-time perspective on Design Patterns, I can recommend "C# Design Patterns" by James Cooper.
For a deeper perspective on the language itself, see Jon Skeet's C# in Depth. It is far and away the best advanced C# book I own.
Finally, there are many questions related to Test-Driven Development here on SO so make sure you browse around!
If you like what you see and decide to use Test Driven development techniques, then you'll either adopt Microsoft's testing library (built into Visual Studio) or you can go for the open source standard NUnit. I use and strongly recommend NUnit. The people at Testdriven.net have built a nice tool that lets you not only run your NUnit tests from within Visual Studio but also provide a great test-coverage analysis tool! I like their tools a lot and use them constantly.
EMail and SMTP-based Messaging
Finally, if you are doing a lot of work using email as a messaging platform, then you'll want to check out the AspNetEmail library. It is far and away the best email platform available. It not only provides things like the automatic processing of ASP.NET web forms, mail merges, etc. etc. it also features technical support with guys who know more about SMTP than anyone else I've ever met. Highly recommended!