You expect to be disappointed when coming face to face with an icon that is almost an archetype, but nothing can really prepare you for the beauty of the Taj Mahal. Built by Shah Jahan as an eternal symbol of his love for his favorite wife, whom he called Mumtaz Mahal ("Jewel of the Palace"), it has immortalized him forever as one of the great architectural patrons of the world. It's not just the perfect symmetry, the ethereal luminescence, the wonderful proportions, or the sheer scale (which is virtually impossible to imagine from staring at its oft-reproduced image), but the exquisite detailing covering every inch of marble that justifies it as a wonder of the world. What appears from afar to be perfectly proportioned white marble magnificence is in fact a massive bejeweled box, with pietra dura adorning the interior and exterior -- said by some to be an Italian technique imported to Agra by Jahangir, but more likely to be a craft originating in Persia. These intricately carved floral bouquets are inlaid with precious stones: agate, jasper, malachite, turquoise, tiger's eye, lapis lazuli, coral, carnelian -- every stone known to man, as well as different shades of marble, slate, and sandstone. Beautiful calligraphy, inlaid with black marble, is carefully increased in size as the eye moves higher, creating an optical illusion of perfectly balanced typography, with the letters the same size from whichever angle you look. Carved relief work, again usually of flowers, which symbolized paradise on earth for the Mughals, decorates much of the interior, while the delicacy of the filigree screens that surround the cenotaph, carved out of a single piece of marble, is simply astounding. The tomb is flanked by two mosques -- one is a prerequisite, but the other is a "dummy" built only in the interests of symmetry; both buildings are worthy of examination in their own right. At the center of it all lies Mumtaz Mahal's cenotaph with the words HELP US OH LORD TO BEAR WHAT WE CANNOT BEAR; Shah Jahan's cenotaph was added later.
Work started in 1641, and the structure took 20,000 laborers 22 years to complete -- legend has it that Shah Jahan cut off the hands of the architect (Persian-born Ustad Ahmad Lahori) and his laborers to ensure that they would never build another, but there is little to substantiate this sensational story.
The Taj changes color depending on the time of day, and many recommend that you witness this by visiting in the morning and evening; however, your ticket is valid for one entry only. Eat a hearty breakfast before you head out (no food is allowed past security), and stay the day, or come in the early morning.
Finally, to understand the symbolism of the Taj, as well as what has been lost since Shah Jahan's day (such as the plunder of the pearl-encrusted silks that covered Mumtaz's cenotaph), it's definitely worth hiring the services of a good (read: official), well-spoken guide. Besides Rajiv Rajawat and Sudhir Agarwal, you can consider arranging a reputable guide through your hotel.
If you're an absolute romantic, you might like to check with your hotel whether or not one of the full moon Taj-viewing experiences is likely to fall on one of the nights during your stay. Since early 2005, the Taj has been open for night viewing for 5 days each lunar cycle: the full moon night and the 2 nights before and after. These after-hours sessions happen between 8pm and midnight and are highly regulated (and certainly no substitute for daytime visits); try to time such a visit for around 10pm.
Note: The Taj is closed on Friday. Your Taj ticket also entitles you to a small discount at the other four major attractions (Agra Fort, Itmad-ud-Daulah, Sikandra, and Fatehpur Sikri), so keep it on hand and show it when paying to enter the others.
Be the First to Arrive -- Get to the Taj entrance at dawn, before it opens, then rush -- run if you must -- straight to the cenotaph chamber (remember to remove your shoes before ascending the marble steps). If you manage to get there first, you will hear what might aptly be described as "the sound of infinity" -- the vibration created by air moving through the huge ventilated dome. As soon as the first visitor walks in, jabbering away, it reverberates throughout the room, and the sacred moment is lost until closing time again.