My only recommendation is to read as much as you can that was written during the Tang dynasty if you want to learn about the Tang dynasty. They wrote a lot, and were prolific commenters and poets. Some of the best historical information out there can be found in works like Steven Owen's three volume study of Tang poetry. (Many of the poets studied were widely influential officials and figures, and their biographies cover huge swaths of highly detailed, recorded history from the time period they were alive.
Poetry was the most essential literary feature of both the Tang and the Song dynasties—civilizations whose very governments were organized and operated with literary technology and along literary principles. That particular collection includes a mass of historical information en route to painting a comprehensive picture of the Tang dynasty and the economic and historical milieus and processes that produced the greatest poets and poetry of China's history.
These kind of studies contain some of the best reading about Tang dynasty history, and that one particularly if you want one book recommendation for the 9th century.
Really I would just read as much as you can written in the Tang and Song, though. There are of course mountains of untranslated literature, but plenty has made it across. Some of it washes up in weird places: a version of a Chinese classic that includes commentary from the period, maybe; or in Thomas Cleary's translations, which contain entire tapestries of Tang and Song historical fabric.^1
There is no single work in English that focuses on the buddhist repression that I'm aware of, if that's what you are looking for. There might be some academic papers out there—often times reading books with good scholarly apparatus can point you to them—but I don't read modern scholarship unless it is attached to translations or studies of original works, generally speaking.^2 The process of learning about the Tang dynasty is one of cobbling together the view points of the citizens of the Tang dynasty themselves, from the writings and historical tracks they left behind. For one book in English with a lot of rich detail, I like Steve Owen's study quite a bit.
The buddhist repression happened smack dab in the middle of one of the most formative centuries for Ch'an, of course. In fact, it occurred at the very midpoint of Joshu's life. It was only for a short period, the closings, but effected the Ch'an communities and Zen Masters who were alive at the time very directly. Because of this there is various information about it strewn across the biographies and Ch'an records as well.
The more Zen texts and studies and collections you read, the more familiar you will become with the Tang dynasty over time. One of the reasons Ch'an survived the repression and flourished afterward was specifically because they were the school of patch robed monks, by the way—to pick up the dropped theme from yesterday.
One of the main Confucian and institutional complaints about buddhism was that it was draining the wealth of the state, and siphoning off the population into parasitic religious institutions that were a drain on the economy and the people. But that's not the patch robed school doing that kind of thing. That's the other guys.
^1 (I have his 5 volume set Classics of Buddhism and Zen as well as his four volume set Collected works of Taoism...he has translated a literal wealth for any student of the era, or of Zen, or of Chinese history and literature. The amount of historical information that accrues to such subjects and texts makes them very useful for building a good understanding of the history. By the time the buddhist repression rolled around in the ninth century, we are watching events unfold whose seeds were planted two and three centuries earlier under the Sui and the time of the early patriarchs—it helps to follow along and understand the historical context as it evolves from one change to the next.
^2 I would absolutely love it if our universities got off their asses and started translating unabridged copies of major historical works from the Tang and Song. When I finally got a good edition of the Spring and Autumn Annals a few years ago it changed my life. The Han Dynasty classic "Records of the Grand Historian" was finally translated in its entirety, too—Burton Watson only did selections—but is still prohibitively expensive for most readers. Needham's Science and Civilization in China is of course an excellent work, and there is a good new reference for Chinese history I love, <em>Chinese History: A New Manual (Fifth Edition)</em>—but I would love nothing more than to sit down with a couple-three ten volume translations of historical works from the Tang and Song dynasties. Sadly, I have no manner of immediately convincing academia that it is worth publishing such stuff other than by writing the funniest comments on Zen literature that I can—so it might take a little while longer before I have their attention.