I've found books from WROX useful (the covers are lame though):
- Beginning ASP.NET 3.5: In C# and VB (Programmer to Programmer)
- Professional ASP.NET 3.5: In C# and VB (Programmer to Programmer)
O'Rielly books aren't bad either:
- ASP.NET in a Nutshell, Second Edition
- Programming ASP.NET, 3rd Edition