Official Class Reference:

http://www.google.com/apis/maps/documentation/reference.html#GTileLayer 

http://www.google.com/apis/maps/documentation/reference.html#GTileLayerOverlay

Examples:

Creating a custom map (discusses copyrights, tile naming, GTileLayer):

http://mapki.com/wiki/Add_Your_Own_Custom_Map

Creating a custom map with bounds check (for limited map coverage), includes code for creating URLs from custom tile splitters: 

http://econym.googlepages.com/custommap.htm

Using GTileLayerOverlay to tint a map white:

http://googlemapsapi.blogspot.com/2006/08/draggable-markers-and.html 

Changing the tone of the map by decreasing opacity of the GMap tile layers & setting page background color:

http://econym.googlepages.com/example_tone.htm

Creating a new Euclidean projection when desiring to use GTileLayer + markers/polylines for flat diagrams:

http://econym.googlepages.com/example_custommapflat.htm

Using Microsoft MapCruncher to produce custom tile layers:

http://www.bdcc.co.uk/GoogleCrunch/Crunch.htm