I have seen lots of books about Han political organizations and foreign policy, but those on the Tang seem to be lacking. Is there a reason for this? What are some of the Tang foreign policy books out there in English?

A few you could check out:

Pan Yihong, "Son of Heaven and Heavenly Qaghan" (deals with relations between Tang and the steppe nomads, though his approach is too sinocentric for me)

Wang Zhenping, "Ambassadors from the Islands of Immortals" (Tang-Japan relations)

Tansen Sen, "Buddhism, Trade and Diplomacy" (Tang-India relations)

Volume 3 of the Cambridge History of China (Sui and Tang, Part 1) also contains the largest amount of English-language material on Tang foreign relations anywhere.

Here are some more....

Backus, C. "The Nan-chao Kingdom and T'ang China's China's Southwestern Frontier", Cambridge, 1981 [read it long time ago, don't remember the content]

Beckwith, Christopher. "Tibetan Empire In Central Asia" [while the author focused on Tibet, you could still learn a thing or two about Tang's foreign policy in central Asia. However, as some members pointed out, the author shown bias toward Chinese sources. Not the easiest book to read]

For Tang political organization:

Finer, C.J "The History of Government" Vol. II ( read chapter 3. Very easy to read)

You should also ck "Warfare in Chinese History", ed. by Van de Ven. It includes a couple of articles on the T'ang period that are very pertinent for this topic.
In Asia Major 3rd series, vol. X you can find the article “Marriage Alliances and Chinese Princesses in International Politics from Han through T’ang” by Pan Yihong (pp.95-135).

I would not use so harsh a word as "biased" for Beckwith's book. I think he merely chooses to take another perspective on the Tang Chinese: instead of glorifying the conquests of the Tang and painting them as having a unprecedented string of military successes, he chooses to show the ugly side of Chinese imperialism and it's many military failures. It is a valid side to show, and a story worth telling.

I do have a problem with his book, though. As I've stated in my review of "The Tibetan Empire In Central Asia", this book has little to do with Tibet. I almost feel cheated by Beckwith; he advertises the book as focusing on Tibet, yet more than (in my estimate) 3/4 of the book focuses on Turkic or Chinese history.

Edouard Chavannes's "History of the Turks (T'ou-kioue)" (in French) has a lot of information about Tang foreign policies in the "Western Regions".

All he really did was selectively translate Zi Zhi Tong Jian word by word into English, throw in few other facts from the two Tang Shu and some other Dun Huang and Islamic sources.
His political science analyzation is also quite weak, he most likely isn't a professional political scientist. Which might be the reason why he tends to seem biased at times by overbloating Tibetan and Arab power while downplaying Turuk influences at times.

The problem with his book isn't his conclusion but his methodology. His most important sources are Zi Zhi Tong Jian and the Tang Shu, but he chose to read between the lines. This is fine except that he made lots of arbitrary assumptions which cannot be backed by sufficient evidence. He also choose to neglect passages in the Zi Zhi Tong Jian which does not support his polemic. Furthermore, his conclusions on the Tibetan domination of the 4 garrisons after 670 has already largely been refuted by the Turfan manuscripts which he never bothered to consult. He attempt to make cogent arguments about things that he is not a professional of; such as Tibetan linguistics and political systems.