The WMP APIs are the only way to access the music library - the Media Center music library is simply a UI on the front of WMP.
To get the album artwork, you need to get the WMCollectionID and wrap it with AlbumArt_{0}_Large.jpg or AlbumArt_{0}_Small.jpg to get the name of the album art file (either full size or thumbnail). Use the path from SourceURL to give you the folder path to the album artwork.
You'll find a complete example of how to do this in the following ZIP file: http://www.digitallivingsolutions.co.uk/downloads/mvpday2007.zip
Cheers,
Andrew