[COLOR="Blue"]Is this the correct glass to fit in the AMANA ART6110W oven door?
Part Number: AP3997466[/COLOR]
Yes this looks like the correct number, though availability may be a problem.

[COLOR="Blue"]Can the oven be used safely for cooking if the inner door glass is cracked?[/COLOR]
I would replace it as the glass will be tempered glass and may explode into a thousand pieces. This could happen when you are cooking and ruin a good meal. Also it may do it when you open the door which could mean glass in your eyes.

[COLOR="Blue"]
Is it an easy job to replace the inner glass.[/COLOR]
Usually it is not that difficult. Remove the door from the unit, the operators manual should tel you how to do this. Then disassemble the door on a table with a couple towels under it.
The following may help
http://www.servicematters.com/maytag_library/docs/16010161.pdf

Do you know of a source for the inner glass or an alternate part number that can be used?

I am afraid I do not have an alternative for you.

All I can suggest is that you open the door assembly up and measure the glass.
Then take the measurement to an appliance parts supplier and see if they can give you one from another unit. I would take the tape measure with you as they may not have one there. If you get a knowledgeable and customer oriented desk person you may get lucky. The glass could be slightly smaller but cannot be larger as it requires some room for expansion when hot.