Official Class Reference

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

http://www.google.com/apis/maps/documentation/reference.html#GMap2

Others References 

Zoom to fit set of markers/all markers (using GLatLngBounds):
http://imagine-it.org/google/debug_bounds.html

Box zoom (include to let you draw a box to zoom in on area):

http://earthcode.com/blog/2006/07/gzoom.html

Using mouse wheel to zoom:
http://maps.forum.nu/gm_mouse_wheel_zoom.html

Gmap2 method enableScrollWheelZoom() enables zooming using a mouse's scroll wheel since 2.78 !

However, you still have to deal with page scrolling.

Auto zoom out from 'sorry' tiles:

http://esa.ilmari.googlepages.com/sorry.htm