Collecting all 4 volumes of Moonlit Bamboo Forest is part of achievement called Spring, White Horse and Moonlight. Player will need to gather all 4 books to complete this collection.
To check which volume that you are lacking, go to Menu > Archive > Books > Scroll until you find Moonlit Bamboo Forest then click + icon, you will find list of volumes that you’ve get.
Here’s locations of all volumes of this book:
Can be found at Granny Rouxin house in Qingce Village.
Volume 2 and 3
Both volumes sold by Jifang of Wanwen Bookhouse for 1500 mora each.
On the table near Verr Goldet in Wangshu Inn.