Ah ok.
The book that contains some information on this is "realistic architectural Visualization with 3ds max and mental ray" but i think its methods are now outdated.
Take a look at the lens effect called "subset of scene". This is a video by Jeff Patton that i am yet to look at:
http://jeffpatton.net/Tips/alpha_mask/alpha_mask.html
You have to unhide this shader and there is more reading somewhere on
http://www.mentalraytips.blogspot.com/
What this basically does is only render out certain objects incase you wanted to change something in a scene and only render out your changes. I guess this could work for render passes but I haven't tried it.
In terms of the render passes for mental ray the basic workflow is to make selection sets based on your different passes. Select the sets you dont want rendered and go object properties > and ucheck 'visible to camera'. This means even though these objects wont render they will still create reflections and shadows on other objects.
Next go to your render dialogue > processing > then click ... next to the greyed out save button. Save your first pass and check the save and merge buttons. Now render your scene. You have just created that first pass. Once thats done click the add button and load the pass you just saved. Now you need to save the next pass under a different name and unhide and the appropirated objects that need to rendered while not forgetting to rehide the objects that you rendered in the previous pass, re-render, add, repeat.
I hope this is clear enough to understand. Sorry :P