It’s been 6 years since I visited Angkor Wat. Absolutely fabulous! I don’t think I’ve ever taken more photos on a trip anywhere else in the world. 
GUIDE BOOK
In addition to a standard guidebook, pick up a copy of this book:
Ancient Angkor, by Michael Freeman
http://www.amazon.com/Ancient-Angkor-Michael-Freeman/dp/0834804263
If you can’t get a copy of it in the States, no worries. This book is available everywhere in Siem Reap-- the hawkers at the temple sites will be waving copies of it to buy. The book has beautiful photos of the temples, suggested itineraries, along w tips on the best time of day to visit. It’ll be very useful to have on the trip and will double as beautiful coffee table book after the trip. FYI, the version of the book I have has a different cover than the one shown on Amazon.
WHAT TO SEE
Besides the obvious temples, it is well worth the effort to see a smaller site: Banteay Srei, a very compact and exceptionally well preserved and beautiful site. Banteay Srei is mentioned in all the guidebooks, but it’s just a bit of a ride to get to it. When we visited it, there was a tour group with nutty women struggling to walk around in high-heeled shoes (hello? it’s an archeological site!). I can’t stress enough the need to wear comfortable shoes. Bring a hat.
WHEN TO GO
Angkor offers a very different attraction in the monsoon season (“winter” months). Yes, it’s rainy, but what’s unique is that the Tonle Sap River (spelling?) reverses flow and forms a huge inland lake that floods the rural villages and turns them into a “floating” village. There are tours to take visitors to see this cultural spectacle.
If you go in the hot summer months, I suggest you visit the temple sites early in the morning, come back to the hotel/town for a swim or siesta, then return to the temple sites in the late afternoon when it’s cooler and the low angle light is great for photos. We visited in the summer. But we have friends who did the winter rainy season and found the floating villages to be very interesting.
MONEY
Some practical money considerations. I recommend only exchanging a minimal amount of cash to Cambodian currency. We made the mistake tof exchanging 100 USD and ended up with a stack of local currency about 4 inches high. The ATMs there dispense USD, and you can pay in USD for practically everything. The local currency is useful for very inexpensive purchases and making change less than 1 USD.
THE TOWN
The town of Siem Reap is fun. We enjoyed eating at the Red Piano and at the Dead Fish. The tourists who visit are culturally minded-- you don’t get the drunken Aussie trailor-trash beach crowd you find in Bali (I love Bali, too, as long as it’s not Kuta Beach).
SOUVENIRS
A nice locally produced item to buy is silk textiles. Bootleg copies of other Cambodia-topic books can be cheaply purchased. I bought some interesting history books.
WHAT ELSE TO DO
There are some super high-end diversions offered: balloon rides, helicopter rides (we didn’t do these because we were leery about the safety standards of the operators), and evening concert/cultural performances right at Angkor Wat.
LOGISTICS
We did the trip independently and simply hired a driver.
FLIGHTS
There are no non-stop flight from the States to Siem Reap, so you will have to transit thru somewhere. Here are some options for carriers:
https://en.wikipedia.org/wiki/Siem_Reap_International_Airport#Airlines_and_destinations
We did a few days in Bangkok and flew via Bangkok Airways. If you need tips on Bangkok, I can help w that, as I visit quite frequently.
Again, it’s been 6 years since I’ve visited, so things may have changed.