Ok, so it will have a strong web programming bias! This list is different to most others, I'm going to assume you basically know how to code in .NET enough to get a senior developer job in most companies. From a very rough poll I'm also going to assume a fair number of you don't have strong University / College CS / Software Engineering backgrounds (I don't). These are the books that over the years I've found useful in filling in / updating bits of my development brain.
NOTE: When it comes down to it, this is MY list...I realize there's some books you just can't believe aren't in the list. This is either because I just don't think they make the cut (too dated like Code Complete / Rapid Development) or too 'daily textbook' like the JavaScript Bible / Programming C#. Feel free to argue / post mini-reviews of books you feel strongly about in the comments (I JUST won't know some of them and that's good too!).
Design / General Practice
- Algorithms in a Nutshell
- Applying Domain Driven Design and Patterns - with examples in C#
- Agile Principles and Practices in C#
- Patterns of Enterprise Application Architecture
- Working Effectively With Legacy Code
- Domain-Driven Design: Tackling Complexity in the Heart of Software
.NET - go deep!
- The CLR via C# (3rd Edition)
- Advanced .NET Debugging (NOT READ THIS YET, PLEASE REVIEW FOR ME!)
TDD, learn, master!
- Test Driven Development By Example
- The Art Of Unit Testing
- xUnit Test Patterns : Refactoring Test Code
The Javascript Triumvirate...you know the syntax, but want to get yourself to godlike status 
- Pro Javascript Techniques
- Javascript: The Good Parts
- Pro Javascript Design Patterns
- Even Faster Web Sites: Performance Best Practices for Web Developers (previous book is in the next section...this one focuses on JS perf / techniques)
Â
CSS / HTML
- Pro CSS and HTML Design Patterns
- CSS: The Missing Manual
- High Performance Web Sites: Essential Knowledge for Front-End Engineers