Looking for books about court culture in Heian Japan and early China (Tang Dynasty and earlier)

Yes, I have got some suggestions about Heian Japan!

The Pillow Book by Sei Shōnagon is the best introduction to Heian court culture I can think of. It's the autobiographical writings of a lady-in-waiting to the Empress Teishi. Shōnagon writes with loving detail about court culture. While some aspects of her portrait are idealized, the notes in the Penguin edition provide a lot of extra context about the difficult political circumstances going on during the period she writes about (Teishi's father died and so she was eclipsed by another empress as the Emperor's primary wife). It makes Shōnagon's glowing reports of the heights of poetic refinement Teishi's salon achieved all the more poignant. Shōnagon also isn't afraid to criticize or poke fun at people and customs of the court. Her attention to human foibles makes the characters from a thousand years ago leap off the page as if they were alive today. The Penguin edition has an excellent introduction and several appendices that include diagrams of the palace layout and clothing as well as an index to important characters, ranks, and terms.

A good supplement to that is The Diary of Lady Murasaki by Murasaki Shikibu. While she's more famous for The Tale of Genji, her diary records her actual opinions of life at court. She served Empress Shōshi, the empress who replaced Teishi. A large chunk of her diary is devoted to writing about Shōshi giving birth to the prince and all the court activities that centred around his birth, both leading up to it and afterwards. While she dislikes court culture more than Sei Shōnagon, comparing their two perspectives is useful for seeing the varying attitudes of women at court.

Another book I'd recommend is Selling Songs and Smiles: The Sex Trade in Heian and Kamakura Japan by Janet R. Goodwin. Heian court nobles regularly patronized singing entertainers who stationed themselves along pilgrimage routes to attract wealthy customers. Goodwin's book analyzes the role of these women (who were often also prostitutes) in Heian culture. Their music, the imayō songs that they would sing, also had a big impact on Heian court culture. You can read more about that in Yung-Hee Kwon's book Songs to Make the Dust Dance: The Ryōjin Hishō of Twelfth-Century Japan.

If you're interested in the religious interplay between Buddhism and Shinto among the Heian elite, I'd recommend The Buddhist Poetry of the Great Kamo Priestess by Edward Kamens. It's a translation of the works of Princess Senshi, who served as the Kamo High Priestess for fifty-six years. Senshi is a great figure to use as a case study for the interaction of Buddhism and Shinto in Heian Japan because although she had a strong personal devotion to the Buddha, as one of the most important Shinto priestesses in the country, she was forbidden from engaging with Buddhism. Looking at how her ritual purity restricted her engagement with the religion she personally wanted to follow provides great insights into the tensions that existed between Shinto and Buddhism in the culture of the Heian court in spite of their relatively peaceful syncretism. The excellent introduction goes through all of this.