recommend a library/API to unzip file in C#

The GPL

http://www.icsharpcode.net/OpenSource/SharpZipLib/

OR the less restrictive Ms-PL

http://www.codeplex.com/DotNetZip

To complete this answer the .net framework has ZipPackage I had less success with it.

If all you want to do is unzip the contents of a file to a folder, and you know you'll only be running on Windows, you can use the Windows Shell object. I've used dynamic from Framework 4.0 in this example, but you can achieve the same results using Type.InvokeMember.

dynamic shellApplication = Activator.CreateInstance(Type.GetTypeFromProgID("Shell.Application"));

dynamic compressedFolderContents = shellApplication.NameSpace(sourceFile).Items;
dynamic destinationFolder = shellApplication.NameSpace(destinationPath);

destinationFolder.CopyHere(compressedFolderContents);

You can use FILEOP_FLAGS to control behaviour of the CopyHere method.

DotNetZip is easy to use. Here's an unzip sample

using (var zip = Ionic.Zip.ZipFile.Read("archive.zip"))
{
   zip.ExtractAll("unpack-directory");
}

If you have more complex needs, like you want to pick-and-choose which entries to extract, or if there are passwords, or if you want to control the pathnames of the extracted files, or etc etc etc, there are lots of options. Check the help file for more examples.

DotNetZip is free and open source.