Keys as per Flora of China and As per efi thread:
What struck me is the spathulate calyx given in the description in your book link (at MS,Aug., 2021/08 Luculia sp.for id.) for Luculia pinceana, which I found at Plants from Northeast Tour 2017- Luculia pinceana from Cherrapunji, Meghalaya. Also lamellate appendages at the base of corolla lobes for this species can be seen at Rubiaceae Week: Sharing Rubiaceae Photo from Arunachal Pradesh and A small tree with white flowers and Tohangam,Lohit,Arunachal Pradesh. 3.1.17



BSI Flora of India Checklist (Distribution):
Luculia gratissima (Wall.) Sweet (Arunachal Pradesh, Assam, Sikkim, West Bengal)
Luculia pinceana Hook. (Arunachal Pradesh, Assam, Meghalaya, Mizoram, Nagaland, West Bengal)

Flora of Peninsular India with keys at species links, if any:
Luculia grandifolia (Cultivated) (E. Himalaya as per POWO)
Luculia gratissima (Cultivated)
Luculia pinceana (Cultivated)

Species with distribution in annotated checklist of Flowering plants of Nepal (Distribution):
Luculia gratissima (Wall.) Sweet (1000-2100 m; Himalaya (Nepal to Assam), Tibet, Indo-China, Yunnan)

Some species found in India with description & keys from Flora of China (Distribution):
Luculia gratissima (Wallich) Sweet (Forests or thickets on mountains; 800-2400 m. Xizang (Mêdog), Yunnan [Bhutan, NE India, Myanmar, Nepal, Thailand, Vietnam])
Luculia pinceana Hooker in Curtis (Forests or thickets on mountain slopes, streamsides in valleys; 600-3000 m. Guangxi, Guizhou, Xizang, Yunnan [India, Myanmar, Nepal, Vietnam])