I will be arriving late in the day and I want to book ahead of time so I don't miss the opportunity.
I don't think it's possible online and should be unnecessary for daytime viewing - the only delay to entry is passing through security checks.
For moonlight viewings it's possible to buy tickets 24h in advance from the ASI offices:
http://asi.nic.in/asi_monu_whs_agratajmahal.asp over 6 years ago
They do close the gates after dark, you can get a local to get you there by camel cart if you wish. I was unaware of the ability to get tickets in advance, however if you bring a local in with you they will cut lines for you. I saved 2 hours wait time doing this. Also I believe Tuesday is washing day... So potentially unavailable for viewing. about 5 years ago