I'm thinking of writing a story set in Korea during this period - specifically during the reign of Queen Seondeok (632-647) in Silla, so I need to find some good resources for things like armor and weapons, clothing, the arts, spread of Buddhism, the life of courtiers, etc. Also names. I'm having trouble finding anything more in-depth than Wikipedia articles.
Searching for books always gets me more recent things like the Joseon era or Korean War, or surveys that only spend a chapter or two on the Three Kingdoms period. I have The New History of Korea by Ki-baik Lee and the Samguk Yusa by Ilyon, which are nice for starters, but I'm going to need much more if I want to be fairly accurate in my story.
If anyone could suggest me a few books to look for or websites, I'd be very grateful. I'd prefer books I can purchase, rather than having to go to a university library (for example), if possible.
Thanks so much for taking the time to help people here.
Without speaking Korean or visiting Gyeongju personally, you've got about all your going to get.
The New History of Korea is a good book, but it's a general history of Korea, not the Three Kingdoms specifically. A History of Korea by Michael Seth is a much larger (and more modern) book than Lee's book, but it's still a general history book.
Searching the library at my university (in Korea), in English, the ONLY book I have found is Overlooked Historical Records of the Three Korean Kingdoms. It appears to be a fuller translation of the Samguk Yusa, so it's not of much use to you. All the other books in the collection are in Korean and, interestingly enough, mostly about Buddhism. I'd take a look at them for you but they're in the Theological library on our other campus, and not available for removal.
The best bet for getting more information is to read about Gyeongju, the capital of Silla. I've got a couple of books at home that I'll add in an edit when I'm home. They're in English, but published in Korea, so they might be a bit tricky to get a hold of them, but I'll see what I can find.
Here is the Gyeongju city guide website
Here is the Cultural Heritage Administration search page
There are loads of treasures in Gyeongju and the surrounding area. You can look through the Gyeongju city guide page to get some pictures and names, then you can use the CHA page to find more information. Just a heads up, Korean websites suck if you aren't using Internet Explorer, so if you have trouble, bite the bullet and load it up.
EDIT
Here are the books I've got that would cover a bit more of what you're looking for:
Kyongju: City of millennial history. (1998). Elizabeth, NJ: Hollym. This is written by the Korean Natinoal Commission for UNESCO. Chapters inclue the Prosperity and Social life of the Capital, Politics and Economy, Archaeological Sites of Kyongju (including The Tombs of Silla), Buddhist Art, Pagodas and Other Monuments, Artifacts, Confucianism and Taoism (written by Lee Ki-baek), as well as others on arts and sciences. Since Kyongju is most famous as the capital of Silla, the bulk of the book focuses on that time period.
Korean cultural heritage: Seen through pictures and names. (2002). Seoul: Sigong Tech. This book is actual two volumes, and it's not Three Kingdom's specific, but it talks about all aspects of Korean culture across all time periods. Of the some 600 pages, there's probably 50 pages worth of Kyongju/Three Kingdoms/Silla information. It's full of great photographs though and a lot of explanations on why things are the way they are. For example, why tombs have gates with spikes on the top, why the sacrifice house in front of the tombs have the stairs on the east and west and not the north and south, the meaning behind the paint scheme you see on all Joseon palaces.